github: Update with better guidance on what's acceptable for src

Tighten and clarify the submission requirements:

• No AI submissions
• No trivial submissions: pull requests have to solve a real problem
• We may timeout requests after a month
• FreeBSD committers (all repos) are responible for merging their commits to the appropriate repo. github can be used to seek approval for such commits (eg a ports committer wants to make a src change).
• Wordsmith a few things to make them clearer to people new to our jargon.
• Not all psuedonyms are created equal, and we may disallow a PR based on that.

Sponsored by: Netflix
Feedback from: emaste
Differential Revision: https://reviews.freebsd.org/D51538