Add PR review guidelines

All contributors should have a set expectation for how their PR will be handled

- [ ] [Add a page](https://github.com/vector-im/element-meta/wiki) with PR review guidelines which include:
- [ ] Who is responsible for the review
- [ ] [Ties in](https://element.discourse.team/t/definition-of-ready-for-element-web-issues/34) with [definition of ready](https://github.com/vector-im/element-meta/issues/1864)
- [ ] [If we use conventional comments](https://element.discourse.team/t/conventional-comments/39/8)
- [ ] Target turnaround times and what they are for community / employees
- [ ] How long the author should wait before following up and with whom

Self assigning, but other contributors can add themselves too and we can work together.

cc @ouchadam