Draft announcements for 10.12 and 11.3

["Adam D. Barratt" <adam@adam-barratt.org.uk>]

Hi,

I've added draft announcements for both of Saturday's point releases to
the publicity announcements repo.

The 10.12 announcement may have an additional section added relating to
the OpenSSL update included in it, but the details are still being
worked out so I've committed the draft without it for now.

Regards,

Adam