doc: add flow bifurcation howto on Linux

doc: add flow bifurcation howto on Linux

Flow Bifurcation is a mechanism which uses features of advanced
Ethernet devices to split traffic between queues. It provides
the capability to let the kernel driver and DPDK driver co-exist
and take advantage of both.

It is achieved by using SR-IOV and the NIC's advanced filtering. This
patch describes Flow Bifurcation and adds the user guide for ixgbe
and i40e NICs.

Signed-off-by: Jingjing Wu <jingjing.wu@intel.com>
Acked-by: John McNamara <john.mcnamara@intel.com>