Add small design for Linux HA integration

This documents the status (or wanted status) of some example ocf modules
I've written for Ganeti. They are far from perfect, but they should be
shipped as a starting point for other people who want to run this to
improve on.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>