PDB-299 Release notes for 1.6.0-rc2 changes

This covers the release notes for 1.6.0-rc2 plus a mention for
prismatic schema as well.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #797 from kbarber/ticket/1.6.x/PDB-299-release_notes_for_rc2

PDB-299 Release notes for 1.6.0-rc2 changes