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 
Why does it need to read the details of the git repo when working with local disk cookbooks? For what purpose? What happens if you are using a non-git SCM? and if you have multiple remotes? I see no need for this.