Add .rubocop.yml

This is the default Hound ruby style config taken from:
https://github.com/thoughtbot/hound/blob/1f03cb64743765801a81c0f7ca9df64a448f1d48/config/style_guides/ruby.yml

These are the checks that Hound currently runs on every pull request.
This config allows people to run rubocop on their local boxes to see
what Hound is going to complain about when they push code.