set release notes for 6.8.4
set release notes for 6.8.4

Co-authored-by: Dagmar Kutz <1a561761b48d63eddc8985df6a6158c391c589a2@artsymail.com>

artsy/eigen,artsy/eigen,artsy/eigen,artsy/eigen,artsy/eigen,artsy/eigen,artsy/eigen