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 
There's not any explicit feature for this (at the moment anyway). I'd bet in berks you can work around this with `eval` to make your own "include" style function. If it's a common pain point we could add that explicitly.