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 https://github.com/docker/docker-registry/issues/623 - basic authentication (implemented by a proxy design in front of the registry service) is validated and will be implemented - this proposal here only covers the alternative key-based, JWT support.

So, I believe this is in line with previous discussions on that subject:

 * allow for third-party to plug themselves into a proxy talking basic auth with the engine
 * support for a key based auth mechanism to be used at least by the Hub, and that could also be used by other people granted they would implement a matching auth service

You are right though, this should be made more clear in that document here.