misc: Added RELEASE-NOTES.md

misc: Added RELEASE-NOTES.md

These release notes will enable users to keep track of the main changes
between gem5 versions. The RELEASE-NOTES.md should also notify the user
of any API changes they should be aware of when using this release of
gem5.

Notes on major changes should be apended to the end of the list as
they are added to the repo.

Change-Id: If75748306a8086b2abbe2f912dbb2ec9f1a6895f
Issue-on: https://gem5.atlassian.net/browse/GEM5-439
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27807
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>