update CONTRIBUTING.md for latest practices

[evancz]

Quick Summary: This file is out of date, so I am trying to merge in the "inbound=outbound" info with the PR expectations that the robot gives.

I am also testing that the recent robot config changes make a comment appear on this PR.

[github-actions[bot]]

Thanks for suggesting these code changes. To set expectations:

• Pull requests are reviewed in batches, so it can take some time to get a response. - Smaller pull requests are easier to review. To fix nine typos, nine specific issues will always go faster than one big one. Learn why here. - Reviewers may not know as much as you about certain situations, so add links to supporting evidence for important claims, especially regarding standards for CSS, HTTP, URI, etc. Finally, please be patient with the core team. They are trying their best with limited resources.