doc: add ring based mempool guide

Add documentation for rte_ring mempool driver.

Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>

doc/guides/mempool/index.rst

@@ -13,3 +13,4 @@ application through the mempool API.
 
     octeontx
     octeontx2
+    ring

doc/guides/mempool/ring.rst

@@ -0,0 +1,34 @@
+..  SPDX-License-Identifier: BSD-3-Clause
+    Copyright(c) 2020 Intel Corporation.
+
+Ring Mempool Driver
+===================
+
+**rte_mempool_ring** is a pure software mempool driver based on the
+``rte_ring`` DPDK library. This is a default mempool driver.
+The following modes of operation are available for the ring mempool driver
+and can be selected via mempool ops API:
+
+- ``ring_mp_mc``
+
+  The underlying **rte_ring** operates in multi-thread producer,
+  multi-thread consumer sync mode.
+
+- ``ring_sp_sc``
+
+  The underlying **rte_ring** operates in single-thread producer,
+  single-thread consumer sync mode.
+
+- ``ring_sp_mc``
+
+  The underlying **rte_ring** operates in single-thread producer,
+  multi-thread consumer sync mode.
+
+- ``ring_mp_sc``
+
+  The underlying **rte_ring** operates in multi-thread producer,
+  single-thread consumer sync mode.
+
+
+For more information about ``rte_ring`` structure, behaviour and available
+synchronisation modes please refer to: :doc:`../prog_guide/ring_lib`.