Add monorepo link to CONTRIBUTING.md
Add link to Youtube about Google monrepo
Also label links with "monorepo" so they are easier to find.
Test Plan: Tested in my own Github repo to see that it redered well, and links work.
Reviewers: deadalnix, #bitcoin_abc, majcosta, PiRK
Reviewed By: #bitcoin_abc, majcosta, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D8796