Documentation to upgrade existing Geo installations

We don't have a documentation about how to upgrade an existing Geo installations.

The steps are simple:

• log into each node (primary and secondaries)
• Upgrade GitLab
• Test primary and secondary node, and check version in each.
• That's it.

Even if the steps are dead simple, we should mention it on https://docs.gitlab.com/ee/gitlab-geo/README.html.