Document PostgreSQL 12 with Geo tests

doc/administration/geo/replication/geo_validation_tests.md

@@ -114,6 +114,19 @@ The following are GitLab upgrade validation tests we performed.
 
 The following are PostgreSQL upgrade validation tests we performed.
 
+### August 2020
+
+[Verify Geo installation with PostgreSQL 12](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5453):
+
+- Description: Prior to PostgreSQL 12 becoming available as an opt-in version in GitLab 13.3,
+  we tested fresh installations of GitLab 13.3 with PostgreSQL 12 enabled and Geo installed.
+- Outcome: Setting up a Geo secondary required manual intervention because the `recovery.conf` file
+  is no longer supported in PostgreSQL 12. We do not recommend deploying Geo with PostgreSQL 12 until
+  the appropriate changes have been made to Omnibus and verified.
+- Follow up issues:
+  - [Update `replicate-geo-database` to support PostgreSQL 12](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5575)
+  - [Remove PostgreSQL 12 check in `replicate-geo-database` for 14.0](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/5576)
+
 ### April 2020
 
 [PostgreSQL 11 upgrade procedure for Geo installations](https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/4975):