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?
You're definitely on the right track, but I guess that it'd be best if me, @jonas054 or @yujinakayama wrote about the implementation details (will be much easier than adjusting this PR lots of times).