Add three more dos/donts to CONTRIBUTING.md

I've seen a couple PRs targeting the wrong branch. Many projects have a
workflow where PRs should hit `develop` or `release` or something, but
Metasploit-Framework wants PRs targeted against `master`.

Also, warn against fixing too much in one PR since those kinds of PRs
are a) harder to validate and b) might be all wrong anyway. We don't
want people committing a bunch of work when the fundamental approach
isn't going to fly.

Also also, don't call it just an "API," but mention that they are mixins.