datapath-windows: Update DESIGN document.

In this patch, we update the design document to reflect the netlink
based kernel-userspace interface implementation and a few other changes.
I have covered at a high level.

Please feel free to extend the document with more details that you think
got missed out.

Signed-off-by: Nithin Raju <nithin@vmware.com>
Acked-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>