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)
This proposal *seems* to step away from some of the earlier discussion about authorization and authentication that occurred in the early registry meetings on IRC, specifically about being more flexible to third party auth solutions.  Is this proposal saying that only one type of authentication (JWT token auth integration with a docker auth server) is supported from the client?  Or is this proposal spec'd just to that type?  

If the latter, it would be good to highlight in this section that this is specifically identifying the JWT token flow and describe Basic / Negotatiate.