"Error: You must set a homebrew user" when running ./run.sh

Ran into an error first time running the packager:

    Error: You must set a homebrew user. at /Users/jcuzella/src/pub/vagrant-installers/modules/homebrew/manifests/params.pp:10 on node vagrant.local

This was probably just left out of the documentation, so I've added a note in the README

 - Add README note about hiera parameter data necessary for build on Darwin