Documentation about upgrading Geo to GitLab 9.0

When upgrading to GitLab 9.0, Geo breaks, due to the change of PostgreSQL's version. The pg_upgraded can't be done automatically on secondary nodes, we need to do the replication backup/restore again.

We need to document the process of migrating to GitLab 9.0. There are two paths:

1. If the customer wants to upgrade PG version and also have the new DR features
   1. Shut down the secondary
   2. Upgrade the primary to GitLab 9.0 and PostgreSQL 9.6
   3. Back up the /var/opt/gitlab/postgresql/data/recovery.conf on the secondary
   4. Upgrade the secondary to GitLab 9.0 and PostgreSQL 9.6.
   5. Rerun the replication step 3 of the Geo secondary docs
   6. On the secondary nodes, an admin must add geo_secondary['enable'] = true to /etc/gitlab/gitlab.rb.
2. If the customer doesn't want to upgrade and deal with that
   1. Use a specific 'skip pg upgrade' flag when reconfiguring

Source: https://gitlab.com/gitlab-org/gitlab-ee/issues/1917#note_25550364