Update doc/advanced/geo/index.md
Adds config to enable GitLab rails on the secondary sites database as part of a Geo setup. As per gitlab-org/gitlab#326236 (closed) there is currently an issue where the GitLab role geo_secondary_role
is behaving differently from the primary. Until a 'correct' way is decided to do this we will need to enable this manual to ensure people don't run into issue when following this guide.