(PDB-686) Add warning about PDB-686 to release notes

This adds a warning about PDB-686 to the release notes so users know how to
work-around it.

This also cleans up the linking in our current release notes, and removes the
warning about Puppet 3.4.x, because we pin against 3.5.1 and greater anyway.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #973 from kbarber/ticket/stable/pdb-686-ubuntu-1204-ruby-193-warnings

(PDB 686) Add warning about PDB-686 to release notes