Add pre-commit to CONTRIBUTING.md

[redphix]

What does this PR do?

Checklist before merging

• If it's a frontend feature, I have ran prettier cd frontend; npm run format. If it's a mobile app feature I ran cd mobile; npm run format.

[redphix]

Also changed the PR template, but not sure if we want it there, because by the time the contributor sees the checklist and hasn't yet installed pre-commit, then it's too late. They will have to install and run the pre-commit hook once manually. Speaking of, will have to run the hooks once so that we have a "clean" state of the repo- pre-commit run --all-files. There are a bunch of whitespace/newlines issues all over the place.

[Reckless-Satoshi]

Looks great!