[MlMt] Test build - release notes update

[Sam Birch]

On Saturday Benny posted r5844, with a comprehensive summary of all changes since r5673. It’s a good read if you’ve been sticking to the release builds and are wondering what you’re missing. It’s also hopefully an indication that the next stable release is getting closer.

I’d still suggest that folks stick to the recommended Big Sur build rather than using random test builds, but of course YMMV.

Here are the release notes:
https://updates.mailmate-app.com/test_release_notes

Cheers,
-sam