Add intro to standalone PostgreSQL

doc/administration/postgresql/standalone.md

 # Standalone PostgreSQL using Omnibus GitLab **(CORE ONLY)**
 
+If you wish to have your database service hosted separately from your GitLab
+application server(s), you can do this using the PostgreSQL binaries packaged
+together with Omnibus GitLab. This is recommended as part of our
+[reference architecture for up to 2,000 users](../reference_architectures/2k_users.md).
+
+## Setting it up
+
 1. SSH into the PostgreSQL server.
 1. [Download/install](https://about.gitlab.com/install/) the Omnibus GitLab
    package you want using **steps 1 and 2** from the GitLab downloads page.