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 
We choose to use filetype extensions because it gets you source code highlighting everywhere that supports it.

The problem with using *only* the filetype extension is that everything else in Chef (roles, environments, etc.) has a Ruby DSL for underlying JSON objects which are interchangeable, e.g., some_role.json and some_role.rb are basically equivalent. So we really need the work "lock" in there somehow. It could be `Policyfile_lock.json`, `PolicyfileLock.json` or something like that. But that will be a bit awkward when we support names other than "Policyfile," which we'll probably need to do to support the monolithic "megarepo" workflow.