Add some more release notes for 0.20.1
Add some more release notes for 0.20.1

Summary: As per title.

Test Plan: Read the release notes.

Reviewers: #bitcoin_abc, deadalnix, Mengerian

Reviewed By: #bitcoin_abc, deadalnix, Mengerian

Subscribers: Mengerian

Differential Revision: https://reviews.bitcoinabc.org/D3970

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