Add 6.6-to-6.7.md update guide

doc/update/6.6-to-6.7.md

+# From 6.6 to 6.7
+
+### 0. Backup
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
+```
+
+### 1. Stop server
+
+    sudo service gitlab stop
+
+### 2. Get latest code
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H git fetch --all
+```
+
+For Gitlab Community Edition:
+
+```bash
+sudo -u git -H git checkout 6-7-stable
+```
+
+OR
+
+For GitLab Enterprise Edition:
+
+```bash
+sudo -u git -H git checkout 6-7-stable-ee
+```
+
+### 3. Update gitlab-shell (and its config)
+
+```bash
+cd /home/git/gitlab-shell
+sudo -u git -H git fetch
+sudo -u git -H git checkout v1.8.0
+```
+
+### 4. Install libs, migrations, etc.
+
+```bash
+cd /home/git/gitlab
+
+# MySQL installations (note: the line below states '--without ... postgres')
+sudo -u git -H bundle install --without development test postgres --deployment
+
+# PostgreSQL installations (note: the line below states '--without ... mysql')
+sudo -u git -H bundle install --without development test mysql --deployment
+
+
+# Run database migrations
+sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
+
+# Clean up assets and cache
+sudo -u git -H bundle exec rake assets:clean assets:precompile cache:clear RAILS_ENV=production
+
+# Update init.d script
+sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
+```
+
+
+### 5. Start application
+
+    sudo service gitlab start
+    sudo service nginx restart
+
+### 6. Check application status
+
+Check if GitLab and its environment are configured correctly:
+
+    sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production
+
+To make sure you didn't miss anything run a more thorough check with:
+
+    sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production
+
+If all items are green, then congratulations upgrade is complete!
+
+## Things went south? Revert to previous version (6.6)
+
+### 1. Revert the code to the previous version
+Follow the [`upgrade guide from 6.5 to 6.6`](6.5-to-6.6.md), except for the database migration 
+(The backup is already migrated to the previous version)
+
+### 2. Restore from the backup:
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:restore RAILS_ENV=production
+```
+If you have more than one backup *.tar file(s) please add `BACKUP=timestamp_of_backup` to the command above.