Add a description of policyfile design and status

I've added a readme for the Policyfile feature so we can point people with questions there for additional info. It describes the overall plan and has a list of not-yet-implemented stuff. The idea is we'll update this as we go.

Thoughts? @opscode/client-engineers 
Yeah, I left that out for now, but, since Policyfiles support roles, they have to allow attributes, and therefore we might as well let you manipulate them in the Policyfile itself (none of that is implemented yet though).