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 
Maybe would be nice to specify where to save the Policifyle: what is its place from the FS/repo point of view. Or at least list some examples. AFAIK, Policyfile can be saved within a *non-chef* application, within a cookbook (similar to `Berksfile`) or even in your `chef-repo/policies` (or similar).