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 
Can we change the attribute values from the Policyfile? (`default_attributes`, `normal_attributes`, ...). And from *policy groups*?

I think it's a good feature of the environments and roles, although currently not used due to versioning problems.