Basic Rubocop instructions

I decided to add a quick note here about how to get rubocop going. Install it from the gem, then run it in the app directory and it will list the issues in your code. I say go to the app folder because then it is a bit more focused on our relevant code, but we could also run it on the tests and initializers too. (The problem then is it is liable to complain about rubygem installed stuff and be a distraction rather than a help.)