[#82150732] add contributing.md

Any feedback?

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Here is what it would look like: https://github.com/Tahi-project/tahi/blob/contributing/contributing.md

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Here is what it looks like when pasted into a PR:

## Pull Request Checklist
- [ ] Ask yourself: is this PR complicated enough that you should write
      integration tests?
- [ ] Do tests pass at every commit? (to preserve ease of `git-bisect`)
- [ ] Ask yourself: do you feel that things are well abstracted?
- [ ] self code review [through `git diff` or the github UI]
- [ ] complete all relevant todos in your code
- [ ] self QA (make sure it works)
- [ ] review from a team-member with knowledge of this part of the codebase
- [ ] **team-member said ship it! (:ship: it)**
- [ ] self QA again Chrome
- [ ] self QA again Firefox
- [ ] self QA again Safari
- [ ] self QA again IE
- [ ] do any changes need to be made to prod before a deploy? List them in
      the `deployer checklist`
- [ ] the tests are passing for this branch on semaphore
- [ ] merge! Celebrate your success!

## Reviewer Checklist
- [ ] syntax consistency with rest of code-base
- [ ] database access patterns / efficiency
- [ ] client-side access patterns / efficiency
- [ ] ember style

## Deployer Checklist
- [ ] **>>list changes here that need to be made to prod before a deploy<<**
- [ ] you are deploying in the morning
- [ ] you are deploying on Monday, Tuesday, Wednesday, or Thursday.
- [ ] you are deploying your own code
- [ ] did the deploy
- [ ] you are watching `heroku logs --tail`
- [ ] you announced the deploy with a few words summary to chatrooms
- [ ] you have opened https://dashboard.heroku.com/apps for rollback
Here are some examples for inspiration:
https://github.com/twitter/code-of-conduct/blob/master/code-of-conduct.md
https://github.com/angular/code-of-conduct/blob/master/CODE_OF_CONDUCT.md
https://github.com/codeforamerica/codeofconduct