Add policyfile generator

Generates a policyfile with comments about usage and link to the POLICYFILE_README

/cc @opscode/client-engineers 
@tyler-ball it's the best we have, I don't know how you could really improve it without rewriting mixlib-cli. You can make it more magical if you `include` a module and then use the module's `Module.included` callback hook, but IMO it's more complicated for more code so not a big help.