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 
As the first paragraph of the section says, the primary purpose is to provide information that might be useful when looking at multiple cookbook revisions with the same name and version number and trying to make sense of them. Additionally, it gives you a head start when debugging issues, especially if you use a workflow that involves uploading cookbooks from `path` sources on a regular basis.