Documentation/networking: Add net DIM documentation

Net DIM is a generic algorithm, purposed for dynamically
optimizing network devices interrupt moderation. This
document describes how it works and how to use it.

Signed-off-by: Tal Gilboa <talgi@mellanox.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>