(PDB-551) Index for versioning policy

We've added a short note about versioning to our docs, this adds a link to the
doc in our index for both 2.0 and master.

I've also corrected a small typo, whereby our release notes were pointing at
the master version instead of 2.0.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #321 from kbarber/ticket/master/pdb-551-versioning-policy

(PDB-551) Index for versioning policy