Add missing release notes for 0.19.8 release

Summary: Adding some info to release notes because this release actually has quite a few commits... Arguably, some of these notes should have been suggested during code review.

Test Plan: Read it.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3323