doc: add QinQ flow to testpmd guide
doc: add QinQ flow to testpmd guide

Add information on validating and creating QinQ flow rules to
the flow rules management section of the Testpmd User Guide.

Signed-off-by: Bernard Iremonger <233eeff376ad0c06f66f06c5522ea3c4feb30d3b@intel.com>
Acked-by: John McNamara <af31595c36a4239b9676e9b3a777d3e2f15a6c4d@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk