Beginnings of code review guidelines

This is not ready to pull yet, of course, but do you think it's a good idea? Worth pursuing?

We can probably borrow ideas from https://github.com/thoughtbot/guides/tree/master/code-review

- [ ] Borrow ideas from Thoughtbot
- [ ] Link to why we do it after the fact, commit by commit

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- [x] HN
- [x] KP
- [ ] JK
- [ ] AR
- [ ] TS
- [ ] VA