Update CONTRIBUTING.md with new link to development group

Summary: Also re-jig the wording in that section a bit.

Test Plan:
Pushed to Github to make sure the Markdown renders well there:
https://github.com/Mengerian/bitcoin-abc/blob/ecash-development-group/CONTRIBUTING.md
Check link, read and make sure it says what we want.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D11472