Added guide for upgrading Postgres using Slony

[ci skip]

CHANGELOG.md

@@ -14,6 +14,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 - Refactor Jira service to use jira-ruby gem
 - Add hover to trash icon in notes !7008 (blackst0ne)
 - Only show one error message for an invalid email !5905 (lycoperdon)
+- Added guide describing how to upgrade PostgreSQL using Slony
 - Fix sidekiq stats in admin area (blackst0ne)
 - Created cycle analytics bundle JavaScript file
 - API: Fix booleans not recognized as such when using the `to_boolean` helper

doc/update/README.md

@@ -85,6 +85,8 @@ possible.
 - [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)
+- [Upgrading PostgreSQL Using Slony](upgrading_postgresql_using_slony.md), for
+  upgrading a PostgreSQL database with minimal downtime.
 
 [omnidocker]: http://docs.gitlab.com/omnibus/docker/README.html
 [source-ee]: https://gitlab.com/gitlab-org/gitlab-ee/tree/master/doc/update