I read through these and found that some of the advice was old and/or
outdated. This change takes a pass at improving the contributing guide.
I'll tackle the local development process next.

In my changes I aimed to:

* Remove outdated information about how to do something (for example
  backports should be done with `cherry-pick`, not `.patch` files).
* Rewrite the easiest, easy, and hard way to better explain each version
  rather than refer to them as easy/hard. I also recommend against the
  rails-dev-box for M1's.
* Remove any language that is flippant or dismissive of contribution
  efforts to better.
* Recommend more modern tools and documentation where appropriate.