Merge branch 'update/upgrade-guide-8.15' into 'master'

[ci skip] Update upgrade guide from GitLab CE to GitLab EE for 8.15

Following the steps from https://gitlab.com/gitlab-org/release-tools/blob/master/doc/release-candidates.md#creating-rc1.

See merge request !959

doc/update/8.15-ce-to-ee.md

+# From Community Edition 8.15 to Enterprise Edition 8.15
+
+This guide assumes you have a correctly configured and tested installation of
+GitLab Community Edition 8.15. If you run into any trouble or if you have any
+questions please contact us at [support@gitlab.com].
+
+### 0. Backup
+
+Make a backup just in case something goes wrong:
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:create RAILS_ENV=production
+```
+
+For installations using MySQL, this may require granting "LOCK TABLES"
+privileges to the GitLab user on the database version.
+
+### 1. Stop server
+
+    sudo service gitlab stop
+
+### 2. Get the EE code
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H git remote add -f ee https://gitlab.com/gitlab-org/gitlab-ee.git
+sudo -u git -H git checkout 8-15-stable-ee
+```
+
+### 3. 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 postgres development test --deployment
+
+# PostgreSQL installations (note: the line below states '--without mysql')
+sudo -u git -H bundle install --without mysql development test --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
+```
+
+### 4. Start application
+
+    sudo service gitlab start
+    sudo service nginx restart
+
+### 5. 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 complete!
+
+## Things went south? Revert to previous version (Community Edition 8.15)
+
+### 1. Revert the code to the previous version
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H git checkout 8-15-stable
+```
+
+### 2. Restore from the backup
+
+```bash
+cd /home/git/gitlab
+sudo -u git -H bundle exec rake gitlab:backup:restore RAILS_ENV=production
+```
+
+[support@gitlab.com]: mailto:support@gitlab.com