doc: Describe backporting process.

doc: Describe backporting process.

This patch documents the backporting process, and provides a walkthrough
for developers who would like to backport upstream Linux patches into
the Open vSwitch tree. Nothing in this documentation should be
surprising or new; it merely puts the existing process into words.

Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>
Acked-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Stephen Finucane <stephen@that.guru>