Proposal: Registry v2 authorization service

Outlines usage for endpoints which will be used by the
docker registry client for registry v2 non-standalone.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
@smarterclayton In the most recent update of the draft spec, the `realm` value is now described as the full URL which you should use to get a token.