doc: net: Add Thread protocol description

Add a documentation page for Thread with a brief description.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

doc/reference/networking/net_tech.rst

@@ -1,11 +1,11 @@
 .. _net_tech:
 
 Networking Technologies
-########################
+#######################
 
 .. toctree::
    :maxdepth: 1
 
    ethernet.rst
    ieee802154.rst
-
+   thread.rst

doc/reference/networking/thread.rst

@@ -0,0 +1,38 @@
+.. _thread_protocol_interface:
+
+Thread protocol
+###############
+
+Overview
+********
+Thread is a low-power mesh networking technology, designed specifically for home
+automation applications. It is an IPv6-based standard, which uses 6LoWPAN
+technology over IEEE 802.15.4 protocol. IP connectivity lets you easily connect
+a Thread mesh network to the internet with a Thread Border Router.
+
+The Thread specification provides a high level of network security. Mesh networks
+built with Thread are secure - only authenticated devices can join the network
+and all communications within the mesh are encrypted. More information about
+Thread protocol can be found at
+`Thread Group website <https://www.threadgroup.org>`_.
+
+Zephyr integrates an open source Thread protocol implementation called OpenThread,
+documented on the `OpenThread website <https://openthread.io/>`_.
+
+Internet connectivity
+*********************
+
+A Thread Border Router is required to connect mesh network to the internet.
+An open source implementation of Thread Border Router is provided by the OpenThread
+community. See
+`OpenThread Border Router guide <https://openthread.io/guides/border-router>`_
+for instructions on how to set up a Border Router.
+
+Sample usage
+************
+
+You can try using OpenThread with the Zephyr Echo server and Echo client samples,
+which provide out-of-the-box configuration for OpenThread. To enable OpenThread
+support in these samples, build them with ``overlay-ot.conf`` overlay config file.
+See :ref:`sockets-echo-server-sample` and :ref:`sockets-echo-client-sample` for
+details.