[CF-metadata] CF Conventions 1.2

[Jonathan Gregory]

Dear Ethan

I agree that a clear statement of the changes is needed. The moderator of the
ticket should do this (that's in the rules already). Apart from it being needed
by software engineers, it is needed first by Velimir to make the changes in
the CF documents. The history of the CF standard (at the end of the doc) links
the tickets which are implemented in each release.

Best wishes

Jonathan