Update docker hub description for gomods/athens

[DonMcNamara]

Describe the bug
The descriptions on the docker hub page for gomods/athens appear to be placeholders. We should update them with some better documentation.

To Reproduce
Steps to reproduce the behavior:

1. Go to: https://hub.docker.com/r/gomods/athens/

Expected behavior
The docker hub descriptions should help the end user get started using Athens. It should probably contain descriptions of what tags we are using, and possibly some information about environment variables (or maybe just a link to the docs?)

We probably don't need something as extensive as this, but the nginx page is a good example.

[arschles]

This is done now. There's a description of how our docker tags are set up, and several links back to GitHub. Closing