Added 8.6 upgrade guide for PostgreSQL users

doc/update/8.5-to-8.6.md

@@ -105,6 +105,23 @@ via [/etc/default/gitlab].
 [Apache templates]: https://gitlab.com/gitlab-org/gitlab-recipes/tree/master/web-server/apache
 [/etc/default/gitlab]: https://gitlab.com/gitlab-org/gitlab-ce/blob/8-6-stable/lib/support/init.d/gitlab.default.example#L37
 
+### 8. Updates for PostgreSQL Users
+
+Starting with 8.6 users using GitLab in combination with PostgreSQL are required
+to have the "pg_trgm" extension enabled for all GitLab databases. If you're
+using GitLab's Omnibus packages there's nothing you'll need to do manually as
+this extension is enabled automatically. Users who install GitLab without using
+Omnibus (e.g. by building from source) have to enable this extension manually.
+To enable this extension run the following SQL command as a PostgreSQL super
+user for _every_ GitLab database:
+
+    CREATE EXTENSION IF NOT EXISTS pg_trgm;
+
+Certain operating systems might require the installation of extra packages for
+this extension to be available. For example, users using Ubuntu will have to
+install the "postgresql-contrib" package in order for this extension to be
+available.
+
 #### Init script
 
 Ensure you're still up-to-date with the latest init script changes: