Added missing release notes

Summary: We've done a lot of backports for this release, so I've noted the important changes here that didn't already have release note callouts.

Test Plan: Read it.

Reviewers: deadalnix, Fabien, Mengerian, markblundeberg, #bitcoin_abc

Reviewed By: Fabien, markblundeberg, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3037