Add instructions on how to add a new cop.

These were things I ran into while I was creating my first cop that I believe will help onboard others.

There's one thing I left blank which is `VariableForce`. I'm not entirely sure, but I believe this does special things , so instead of getting a `node` you'd get a few nodes that match a type of group? For instance all the nodes in an assignment. Does anybody want to point me in the right direction here?
Cops that are not based on style guide rules should be disabled (normally) by default.