Add cardboard + improve documentation

The module does not really _require_ the `boxen` puppet module in a way that it would fail without it. It may check content of some boxen variables/facts, but if they don't exist, it will fall back to defaults.

https://github.com/boxen/puppet-brewcask/blob/master/lib/puppet/provider/package/brewcask.rb#L19
https://github.com/boxen/puppet-brewcask/blob/master/lib/puppet/provider/package/brewcask.rb#L83