docs: cutting a release

continuing the discussion from: https://github.com/gomods/athens/pull/547#discussion_r212680314

We need to outline the steps required for a maintainer to cut a new release of either the proxy or Olympus.

Start with the discussion in pull request #547, linked from this issue, to recover the release concerns already raised. Document the steps a maintainer needs to cut a new release for both the proxy and Olympus, with completion meaning either release path is sufficiently outlined for someone to follow.

Written by the indexing model from the issue text.