Geo: Document steps to enable FDW if running from before 10.4

We need to document the steps to enable FDW when a user has been running Geo since before 10.4. In 10.6, we began showing a warning when FDW is not enabled. This is leading to some confusion with customers that something is wrong after upgrading to 10.6.

Potential steps:

  1. Run gitlab-ctl pg-password-md5 gitlab on primary
  2. Set postgresql['sql_user_password'] in gitlab.rb
  3. Reconfigure and restart postgres
  4. On secondary, set postgresql['sql_user_password']
  5. On secondary, set geo_secondary['db_fdw'] = true in gitlab.rb
  6. On secondary, set postgresql['md5_auth_cidr_addresses'] = ['<secondary_ip>/32']
  7. Reconfigure and restart postgres

Customer ticket -> https://gitlab.zendesk.com/agent/tickets/93784 (internal)

/cc @stanhu @brodock @jramsay

Edited by Aric Buerer