Add user cohorts and usage ping documentation

doc/administration/usageping/usage_ping_cohorts.md

+# Usage ping and cohorts
+
+> **Notes:**
+- [Introduced][ee-380] in GitLab EE 8.10 and [added to CE][ce-23361] in GitLab 9.1.
+
+## Usage ping
+
+In order for GitLab to become a data-driven company, where deciding what to
+build next is driven by how users are using the product, GitLab needs to collect
+usage data. This is done via a feature called usage ping.
+
+Usage ping is a weekly, anonymous JSON sent to GitLab, containing several
+metrics on how people use specific features.
+
+You can see at any given point in time the content of the JSON file that will be
+sent in the Administration panel of your GitLab installation. This feature can
+also be deactivated at any given time.
+
+The information that is sent contains the total number of:
+* Comments
+* Groups
+* Users
+* Projects
+* Issues
+* Labels
+* CI builds
+* Snippets
+* Milestones
+* Todos
+* Pushes
+* Merge requests
+* Environments
+* Triggers
+* Deploy keys
+* Pages
+* Project Services
+* Issue Boards
+* CI Runners
+* Deployments
+* Geo Nodes
+* LDAP Groups
+* LDAP Keys
+* LDAP Users
+* LFS objects
+* Protected branches
+* Releases
+* Remote mirrors
+* Web hooks
+
+More will be added over time. The goal of this ping is to be as light as
+possible, so it won't have any performance impact on your installation when
+calculation is made.
+
+### Deactivate the usage ping
+
+By default, usage ping is opt-out. If you want to deactivate, go to the Settings
+page of your administration panel and uncheck the Usage ping checkbox.
+
+![User cohort example](img/cohorts.png)
+
+## Cohorts
+
+As a benefit of having the usage ping active, GitLab lets you analyze the user's
+activities of your GitLab installation. Under [LINK], when the usage ping is
+active, GitLab will show the monthly cohorts of new users and their activities
+over time.
+
+How do we read user cohorts table? Let's take an example with the following user
+cohorts.
+
+[IMAGE]
+
+For the cohort of June 2016, 163 users have been created on this server. One
+month after, in July 2016, 155 users (or 95% of the June cohort) are still
+active. Two months after, 139 users (or 85%) are still active. 9 months after,
+we can see that only 6% of this cohort are still active.
+
+How do we measure the activity of users? GitLab considers a user active if:
+* the user signs in
+* the user has a git activity (wether push or pull).
+
+### Setup
+
+1. Activate the usage ping as defined in [LINK]
+2. Go to [LINK] to see the user cohorts of the server