Access.md added.

Some thoughts around identity and access management.  
Ideas still rough, but trying to share early.  Comments welcomed.

Some of this maybe should go into docs/security.md
Thinking through the things we've said for authz, namespaces, and projects, I'm starting to think that users is a concept (like projects) that sits above or alongside Kubernetes, but not in it.  In a simple example, I have three layers:

* Authentication proxy
* Authorization proxy
* API Server

The authentication proxy loads data about a user, and passes it along.  The authorization proxy takes that user information and checks a policy, and then applies or denies the request.  The API server is completely ignorant of both policy, users, or authentication mechanisms, and simply processes the request.

In this model projects, ACL, authn, and authz, users, and membership all occur outside of the core system.  So there would be no need for K8s to know about user accounts.  That seems like something that benefits from being in "Initial Features".

The one gap would be audit - associating a created / modified API resource with an identity.  We could state that a K8s apiserver could take as input on any request an opaque identity string that would be recorded as the initiator of the action - it would be the responsibility of the auth(n) proxy to set that data on the incoming request in a way that the apiserver will assign it.