[10.0.0 Release] Please help writing release notes!

Hello everyone,

We're getting closer to the final release, and as usual I'd like to
ask you all to help writing release notes.

When the release happens, the first thing people tend to look at are
the release notes, so they're a great opportunity to highlight the
work that's been done over the past six months. As you can see from
the current notes (linked from
https://prereleases.llvm.org/10.0.0/#rc2), there's plenty of room for
improvement.

If you know anything that's worth adding, please commit directly to
ReleaseNotes.rst on the branch, upload a patch to Phabricator, or even
just send me an email with some text and I'll happily commit it for
you.

Many thanks,
Hans