NCBC-345: Update Readme.mdown to reflect changes in our versioning policy

Change-Id: I4c61f547c53efb2972700c458267dd389084b9c8
Reviewed-on: http://review.couchbase.org/31044
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>