Added some notes for the 0.19.5 release

Summary: Filling in some extra notes so they aren't so sparse.

Test Plan: Read it.

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

Reviewed By: Fabien, Mengerian, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D2910