[WIP] Add more content to the Contributing Guide
Summary: The goal of this is to make the Contributing Guide full-featured and make it a useful reference for newcomers.
Test Plan: Something something world's largest rubber ducky something something sombreros.
Reviewers: lynorian, wxl
Reviewed By: lynorian
Subscribers: lynorian, wxl
Differential Revision: https://phab.lubuntu.me/D35