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