[NET]: [DOC] Multiqueue hardware support documentation

Add a brief howto to Documentation/networking for multiqueue.  It
explains how to use the multiqueue API in a driver to support
multiqueue paths from the stack, as well as the qdiscs to use for
feeding a multiqueue device.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>