Split into sections, port `downgrade_ee_to_ce` doc from EE

doc/downgrade_ee_to_ce/README.md

+# Downgrading from EE to CE
+
+If you ever decide to downgrade your Enterprise Edition back to the Community
+Edition, there are a few steps you need take before installing the CE package
+on top of the current EE package, or, if you are in an installation from source,
+before you change remotes and fetch the latest CE code.
+
+## Disable Enterprise-only features
+
+First thing to do is to disable the following features.
+
+### Authentication mechanisms
+
+Kerberos and Atlassian Crowd are only available on the Enterprise Edition, so
+you should disable these mechanisms before downgrading and you should provide
+alternative authentication methods to your users.
+
+### Git Annex
+
+Git Annex is also only available on the Enterprise Edition. This means that if
+you have repositories that use Git Annex to store large files, these files will
+no longer be easily available via Git. You should consider migrating these
+repositories to use Git LFS before downgrading to the Community Edition.
+
+### Remove Jenkins CI Service entries from the database
+
+The `JenkinsService` class is only available on the Enterprise Edition codebase,
+so if you downgrade to the Community Edition, you'll come across the following
+error:
+
+```
+Completed 500 Internal Server Error in 497ms (ActiveRecord: 32.2ms)
+
+ActionView::Template::Error (The single-table inheritance mechanism failed to locate the subclass: 'JenkinsService'. This
+error is raised because the column 'type' is reserved for storing the class in case of inheritance. Please rename this
+column if you didn't intend it to be used for storing the inheritance class or overwrite Service.inheritance_column to
+use another column for that information.)
+```
+
+All services are created automatically for every project you have, so in order
+to avoid getting this error, you need to remove all instances of the
+`JenkinsService` from your database:
+
+**Omnibus Installation**
+
+```
+$ sudo gitlab-rails runner "Service.where(type: 'JenkinsService').delete_all"
+```
+
+**Source Installation**
+
+```
+$ bundle exec rails runner "Service.where(type: 'JenkinsService').delete_all" production
+```
+
+## Downgrade to CE
+
+After performing the above mentioned steps, you are now ready to downgrade your
+GitLab installation to the Community Edition.
+
+**Omnibus Installation**
+
+To downgrade an Omnibus installation, it is sufficient to install the Community
+Edition package on top of the currently installed one. You can do this manually,
+by directly [downloading the package](https://packages.gitlab.com/gitlab/gitlab-ce)
+you need, or by adding our CE package repository and following the
+[CE installation instructions](https://about.gitlab.com/downloads/).
+
+**Source Installation**
+
+To downgrade a source installation, you need to replace the current remote of
+your GitLab installation with the Community Edition's remote, fetch the latest
+changes, and checkout the latest stable branch:
+
+```
+$ git remote set-url origin git@gitlab.com:gitlab-org/gitlab-ce.git
+$ git fetch --all
+$ git checkout 8-x-stable
+```
+
+Remember to follow the correct [update guides](../update/README.md) to make
+sure all dependencies are up to date.

doc/update/README.md

-Depending on the installation method and your GitLab version, there are multiple update guides. Choose one that fits your needs.
+# Updating GitLab
+
+Depending on the installation method and your GitLab version, there are multiple
+update guides.
+
+There are currently 3 official ways to install GitLab:
+
+- Omnibus packages
+- Source installation
+- Docker installation
+
+Based on your installation, choose a section below that fits your needs.
+
+---
+
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+**Table of Contents**  *generated with [DocToc](https://github.com/thlorenz/doctoc)*
+
+- [Omnibus Packages](#omnibus-packages)
+- [Installation from source](#installation-from-source)
+- [Installation using Docker](#installation-using-docker)
+- [Upgrading between editions](#upgrading-between-editions)
+    - [Community to Enterprise Edition](#community-to-enterprise-edition)
+    - [Enterprise to Community Edition](#enterprise-to-community-edition)
+- [Miscellaneous](#miscellaneous)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
 ## Omnibus Packages
 
-- [Omnibus update guide](http://doc.gitlab.com/omnibus/update/README.html)
-  contains the steps needed to update a GitLab
-  [package](https://about.gitlab.com/downloads/).
+- The [Omnibus update guide](http://doc.gitlab.com/omnibus/update/README.html)
+  contains the steps needed to update an Omnibus GitLab package.
 
 ## Installation from source
 
-- [The individual upgrade guides](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/update) are for those who have installed GitLab from source.
-- [The CE to EE update guides](https://gitlab.com/gitlab-org/gitlab-ee/tree/master/doc/update) are for subscribers of the Enterprise Edition only. The steps are very similar to a version upgrade: stop the server, get the code, update config files for the new functionality, install libs and do migrations, update the init script, start the application and check the application status.
+- [Upgrading Community Edition from source][source-ce] - The individual
+  upgrade guides are for those who have installed GitLab CE from source.
+- [Upgrading Enterprise Edition from source][source-ee] - The individual
+  upgrade guides are for those who have installed GitLab EE from source.
 - [Patch versions](patch_versions.md) guide includes the steps needed for a
-  patch version, eg. 6.2.0 to 6.2.1.
+  patch version, eg. 6.2.0 to 6.2.1, and apply to both Community and Enterprise
+  Editions.
+
+## Installation using Docker
+
+GitLab provides official Docker images for both Community and Enterprise
+editions. They are based on the Omnibus package and instructions on how to
+update them are in [a separate document][omnidocker].
+
+## Upgrading between editions
+
+GitLab comes in two flavors: [Community Edition][ce] which is MIT licensed,
+and [Enterprise Edition][ee] which builds on top of the Community Edition and
+includes extra features mainly aimed at organizations with more than 100 users.
+
+Below you can find some guides to help you change editions easily.
+
+### Community to Enterprise Edition
+
+>**Note:**
+The following guides are for subscribers of the Enterprise Edition only.
+
+If you wish to upgrade your GitLab installation from Community to Enterprise
+Edition, follow the guides below based on the installation method:
+
+- [Source CE to EE update guides][source-ee] - Find your version, and follow the
+  `-ce-to-ee.md` guide. The steps are very similar to a version upgrade: stop
+  the server, get the code, update config files for the new functionality,
+  install libraries and do migrations, update the init script, start the
+  application and check its status.
+- [Omnibus CE to EE][omni-ce-ee] - Follow this guide to update your Omnibus
+  GitLab Community Edition to the Enterprise Edition.
+
+### Enterprise to Community Edition
+
+If you need to downgrade your Enterprise Edition installation back to Community
+Edition, you can follow [this guide][ee-ce] to make the process as smooth as
+possible.
 
 ## Miscellaneous
 
 - [MySQL to PostgreSQL](mysql_to_postgresql.md) guides you through migrating
   your database from MySQL to PostgreSQL.
-- [MySQL installation guide](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/install/database_mysql.md) contains additional information about configuring GitLab to work with a MySQL database.
+- [MySQL installation guide](../install/database_mysql.md) contains additional
+  information about configuring GitLab to work with a MySQL database.
 - [Restoring from backup after a failed upgrade](restore_after_failure.md)
+
+[omnidocker]: http://doc.gitlab.com/omnibus/docker/README.html
+[source-ee]: https://gitlab.com/gitlab-org/gitlab-ee/tree/master/doc/update
+[source-ce]: https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/update
+[ee-ce]: ../downgrade_ee_to_ce/README.md
+[ce]: https://about.gitlab.com/features/#community
+[ee]: https://about.gitlab.com/features/#enterprise
+[omni-ce-ee]: http://doc.gitlab.com/omnibus/update/README.html#from-community-edition-to-enterprise-edition