[doc] Add two articles to CONTRIBUTING.md

Summary: Good articles pointed out by Amaury

Test Plan: Tested link at: https://github.com/Mengerian/bitcoin-abc/blob/step-by-step/CONTRIBUTING.md

Reviewers: deadalnix, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10483