doc: add net null PMD guide

Net null PMD was missing documentation, adding it.

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>

doc/guides/nics/index.rst

@@ -46,6 +46,7 @@ Network Interface Controller Drivers
     netvsc
     nfb
     nfp
+    null
     octeontx
     octeontx2
     pfe

doc/guides/nics/null.rst

@@ -0,0 +1,39 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2020 Intel Corporation.
+
+NULL Poll Mode Driver
+=====================
+
+NULL PMD is a simple virtual driver mainly for testing. It always returns success for all packets for Rx/Tx.
+
+On Rx it returns requested number of empty packets (all zero). On Tx it just frees all sent packets.
+
+
+Usage
+-----
+
+.. code-block:: console
+
+   $RTE_TARGET/app/testpmd -l 0-3 -n 4 --vdev net_null0 --vdev net_null1 -- -i
+
+
+Runtime Config Options
+----------------------
+
+- ``copy`` [optional, default disabled]
+
+ It copies data of the packet before Rx/Tx. For Rx it uses another empty dummy mbuf for this.
+
+.. code-block:: console
+
+   $RTE_TARGET/app/testpmd -l 0-3 -n 4 --vdev "net_null0,copy=1" -- -i
+
+- ``size`` [optional, default=64 bytes]
+
+ Custom packet length value to use.r
+ If ``copy`` is enabled, this is the length of copy operation.
+
+.. code-block:: console
+
+   $RTE_TARGET/app/testpmd -l 0-3 -n 4 --vdev "net_null0,size=256" -- -i
+