Updated CONTRIBUTING.md

Created by: ascent12

Changed the contribution guidelines to be more consistent with the style that wlroots has evolved into, partially mentioned in #93 (closed). This is my current interpretation of what the style is actually supposed to be. Correct me if I'm wrong.

I've removed some of the things which was duplicating information from the kernel style, and am mainly highlighting the differences from it. We just defer to the kernel style in cases not mentioned here.

I removed the code example, as it had references to very sway specific things, and it didn't actually follow the style being talked about. Not to mention that I think the code was probably a bit too complicated for an example.
I looked around the wlroots codebase for a replacement, but I couldn't find a reasonably self-contained example that showcased the style and not have some other problem with it. We can add the example back when the code gets cleaned up a little bit.