[Release-testers] RELEASE_TESTERS.TXT

[Renato Golin via Release-testers]

Folks,

I've added a new file with the names of all volunteered release
testers so far. But I don't want to commit that file without the
explicit approval from everyone on that list.

https://reviews.llvm.org/D22037

There's also some changes to the release document. I'll be continuing
to re-factor those in the next few days. Feel free to add any comments
to those as well.

Tom, Hans,

Your names are also marked as release managers. I know you have
accepted those positions publicly, but it would be good if you could
ack on that review as well.

Thank you!
--renato