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 
I personally find it confusing to use the double extension. IMHO we had to choose between these two:

1. `Policyfile.rb` -> `Policyfile.json`: like the *metadata.rb* (I would choose this for consistency).
2. `Policyfile` -> `Policyfile.lock`: the *Gemfile*, *Berksfile* way.