Added README for generating chainparams constants

Summary: Now that the tooling for this exists, we can simply the release-process notes.

Test Plan: Read it and try it out for yourself.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4157