Add PostgreSQL settings for GitLab.com
This documents the settings/values for PostgreSQL that we currently use for GitLab.com. Do note that these settings are specific to GitLab.com (e.g. the memory we have available in our servers), you can't just copy-paste them into another environment.
Fixes https://gitlab.com/gitlab-com/infrastructure/issues/2140
Edited by Yorick Peterse