Add pull request template with contribution notice and review checklist (#582)

* Add PR template with contribution notice and review checklist

- Add `.github/pull_request_template.md`
- Set expectations that external PRs may not be reviewed or merged
- Require clear change rationale plus UI before/after media when applicable

* Add contribution policy notice and link detailed guidelines

- add CONTRIBUTING.md with clear expectations for external PRs
- update README to direct would-be contributors to CONTRIBUTING.md first

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TBTheo Browne committed 7 months ago5815ba2parent 5de8e9d