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
Fix reference to PuppetDB 1.6.x in index.markdown

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #964 from kbarber/maint/stable/remove-version-16x-references

Maint/stable/remove version 16x references
(PDB-551) Include a versioning policy

This patch adds a new document for outlining our ongoing software & API
versioning policy.

Signed-off-by: Ken Barber <ken@bob.sh>
(PDB-551) Some minor cleanups to the versioning policy

Signed-off-by: Ken Barber <ken@bob.sh>
(PDB-551) Reduce wording based on feedback

Reduced the amount of explicit statements based on feedback, simplified
the wording around experimental a lot.

Also provided a link to the commands versioning page.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #953 from kbarber/ticket/stable/pdb-551-versioning-policy

(PDB-551) Versioning policy
Unpin the version of beaker

We had pinned beaker previously because we were waiting for some of our new EC2
customisations to be merged in and released. This has been done now.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #965 from kbarber/maint/1.6.x/unpin-beaker

Unpin the version of beaker
Merge branch '1.6.x' into stable

* 1.6.x:
  Unpin the version of beaker
Merge branch 'stable'

* stable:
  Unpin the version of beaker
  (PDB-551) Reduce wording based on feedback
  (PDB-551) Some minor cleanups to the versioning policy
  (PDB-551) Include a versioning policy
  Fix reference to PuppetDB 1.6.x in index.markdown
  Update release notes with more upgrade warnings