Geo - Add required extensions for secondary tracking databases

doc/install/postgresql_extensions.md

@@ -9,7 +9,7 @@ info: To determine the technical writer assigned to the Stage/Group associated w
 This guide documents how to manage PostgreSQL extensions for installations with an external
 PostgreSQL database.
 
-The following extensions must be loaded into the GitLab database:
+You must load the following extensions into the main GitLab database (defaults to `gitlabhq_production`):
 
 | Extension    | Minimum GitLab version |
 |--------------|------------------------|
@@ -17,6 +17,13 @@ The following extensions must be loaded into the GitLab database:
 | `btree_gist` | 13.1                   |
 | `plpgsql`    | 11.7                   |
 
+If you are using [GitLab Geo](https://about.gitlab.com/solutions/geo/), you must load the following 
+extensions into all secondary tracking databases (defaults to `gitlabhq_geo_production`):
+
+| Extension    | Minimum GitLab version |
+|--------------|------------------------|
+| `plpgsql`    | 9.0                    |
+
 In order to install extensions, PostgreSQL requires the user to have superuser privileges.
 Typically, the GitLab database user is not a superuser. Therefore, regular database migrations
 cannot be used in installing extensions and instead, extensions have to be installed manually