Add some docs for Docker Registry configuration - Relatively important for anyone installing from source

There have been a few issues created because of the lack of documentation for how to configure Docker Registry when installing from source, specifically what the `realm` needs to be and the `service`

These 2 bits of info will stop a lot of headaches 👍 

Issues:
https://gitlab.com/gitlab-org/gitlab-ce/issues/17957
https://gitlab.com/gitlab-org/gitlab-ce/issues/17991

See merge request !4738