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)
Should this be "Bearer" rather than "Token" to be compatible with [RFC 6750](https://tools.ietf.org/html/rfc6750)?