[9.0.0 Release] Please help writing release notes

Hi everyone,

The end of the release process is getting close, and as usual I'd like
to ask you all to help writing release notes.

As you can see from the release notes for rc2 here:
https://prereleases.llvm.org/9.0.0/#rc2 there is plenty of room for
more notes :slight_smile:

When the release happens, the first thing people look at are these
notes, so it's a great opportunity to show what we've been doing for
the past six months.

If you know anything that's worth adding, please commit to the release
notes directly on the branch, or send me a patch, or even a plain
email with some text and I'll happily commit for you.

I don't know whether it's possible to commit to the release branch
from the Git monorepo, but it's easy to do with Subversion; for

$ svn co --non-recursive
$ cd llvm90docs
(edit ReleaseNotes.rst)
$ svn commit

Many thanks,