Add Trunk Based Development link to CONTRIBUTING
Summary: Another doc to help explain good development practices
Test Plan:
Checked on my Github the it looks good and link works
https://github.com/Mengerian/bitcoin-abc/blob/trunk/CONTRIBUTING.md
Reviewers: deadalnix, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D9046