Summary:
To help new developers get on board, let's provide a `CONTRIBUTING.md`
doc.

Test Plan: Proofread with your eyeballs.

Reviewers: thcipriani, demon, hashar, mmodell, zeljkofilipin, greg, Jrbranaa, #release-engineering-team

Reviewed By: thcipriani, hashar, #release-engineering-team

Tags: #release-engineering-team

Differential Revision: https://phabricator.wikimedia.org/D993