Merge branch 'slony-upgrade-guide' into 'master'

Added guide for upgrading Postgres using Slony

## What does this MR do?

This adds a guide for upgrading PostgreSQL using Slony.

## Are there points in the code the reviewer needs to double check?

Spelling, grammar, etc

## Why was this MR needed?

This process is not documented and we'll need to provide steps on how to do this before we start shipping PostgreSQL 9.6 with omnibus-gitlab.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc @pcarranza @marin @axil

See merge request !6711

CHANGELOG.md

@@ -17,6 +17,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 - Add hover to trash icon in notes !7008 (blackst0ne)
 - Hides project activity tabs when features are disabled
 - 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)
 - Added label description as tooltip to issue board list title
 - Created cycle analytics bundle JavaScript file

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