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 
Can Policyfiles include other Policyfiles recursively? (like roles). One problem I see is common in Berksfiles is that many harcoded git/directory paths must be copy&pasted in many Berksfiles. The same problem can happen with Policyfiles.