Update release notes with more upgrade warnings

I've nabbed the upgrade warnings from the announcement and put them here to
make sure our notices aren't missed. It seems not everyone reads the
announcement and we really should have the warnings here anyway for
permanence.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #963 from kbarber/maint/stable/expand_release_notes

Update release notes with more upgrade warnings