ovn: Add initial design documentation.

ovn: Add initial design documentation.

This commit adds preliminary design documentation for Open Virtual Network,
or OVN, a new OVS-based project to add support for virtual networking to
OVS, initially with OpenStack integration.

This initial design has been influenced by many people, including (in
alphabetical order) Aaron Rosen, Chris Wright, Gurucharan Shetty, Jeremy
Stribling, Justin Pettit, Ken Duda, Kevin Benton, Kyle Mestery, Madhu
Venugopal, Martin Casado, Natasha Gude, Pankaj Thakkar, Russell Bryant,
Teemu Koponen, and Thomas Graf.  All blunders, however, are due to my own
hubris.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>