net: add Documentation/networking/scaling.txt

Describes RSS, RPS, RFS, accelerated RFS, and XPS.

This version incorporates comments by Randy Dunlap and Rick Jones.
Besides text cleanup, it adds an explicit "Suggested Configuration"
heading to each section.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-By: Rick Jones <rick.jones2@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>