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