Add Kent Beck link to CONTRIBUTING.md
Summary: Amaury posted this link in the Slack, so good to put it here also.
Test Plan: I pushed to my personal Gihub repo, and checked that the markdown renders properly, and the link works.
Reviewers: deadalnix, #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7034