Closes #5007 - Add PR guidelines to README.

[liuche]

Adding some PR guidelines that we discussed to the README. This also links to the shared android PR guidelines docs.

This is definitely only a first step!

I'd also consider: linking from the description of in the Fenix repo (at the top, where there's lots of visibility) to either this new PR section of the README, or copying the guidelines there directly.

I'll also send an email out, once we decide to merge this.

Open questions:

• Should we discuss that we prioritize our roadmap (code and review) as part of this change?
• Are we going to timebox the amount of time we spend on non-sprint reviews?

Pull Request checklist

• Quality: This PR builds and passes detekt/ktlint checks (A pre-push hook is recommended)
• Tests: This PR includes thorough tests or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features

[codecov-io]

Codecov Report

Merging #5008 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             master   #5008   +/-   ##
========================================
  Coverage      8.72%   8.72%           
  Complexity      192     192           
========================================
  Files           231     231           
  Lines          9137    9137           
  Branches       1326    1326           
========================================
  Hits            797     797           
  Misses         8287    8287           
  Partials         53      53

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[sblatz]

r+

Can we also include a comment mentioning to look at help wanted tickets? Those are ones that we generally think are great for contributors to pick up 😄

[sblatz]

Can we make a note here about also including self-comments on PR's when necessary rather than just a summary of the work? I find these to be more useful :)

[sblatz]

nit: Use micro-commits. This makes it easier and faster to review.

[liuche]

I skipped the help-wanted mention bc it's already listed in under "good first issues" as well as in the CONTRIBUTING.md file we link to.

I also added a part explaining why we might be slow handling PRs, which Jeff brought up.

Some other things we could consider are making this more step-by-step, but in light of landing stuff early and iterating, I'm going to merge this in. Open to more feedback or re-opening the issue.