Refocus CONTRIBUTING.rst on external contributors - FLOC-959

Fixes FLOC-959.
People will do whatever they want no matter what we write.  I think we might as well ask for the ideal experience.  I don't want people to think we're going to be cool to merge whatever undescribed 5000 line PR they throw our way.

What do we actually want?

  * A description of what the change is actually going to be and why it's necessary
  * A chance to discuss the design of any major changes before we have to reject hours or days or more of work as totally unusable
  * PRs that focus on one issue, not that combine many unrelated changes into one