Adding basic CONTRIBUTING
A tiny refactoring.

I generally don't like the 'extract into a module' refactoring, but this
code was duplicated between the two. I only noticed because of Code
Climate, and it turns out that semantically, there was some code which
was technically different. It's the `Array()` part of this request.

I glanced at a few other methods, and it seems like there's a lot of
duplication in general. I bet this module could actually turn into some
kind of superclass at some moment in the future, but right now, I'm just
trying to get a handle on bits of things.