Add Wi-SUN introduction

Author: Arto Kinnunen

docs/reference/technology/mesh/wisun_intro.md

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+### Introduction to Wi-SUN networking stack
+Initial version of Mbed Wi-SUN networking stack was introduced in Mbed OS release 5.12.
+
+More details about Wi-SUN can be found from [Wi-SUN Alliance site](https://www.wi-sun.org).
+
+#### Why to use Wi-SUN Alliance FAN (Field Area Network) profile
+Wi-SUN FAN is based on open standards from IEEE802, IETF, ANSI/TIA and ETSI.
+
+Wi-SUN FAN operates on license-exempt sub-GHz RF band and is using frequency hopping to lower interference. Therefore Wi-SUN is well suited for outdoor installations and dense urban neighborhoods.
+
+The Mbed Wi-SUN stack is built on IPv6 over Low power Wireless Personal Area Networks (6LoWPAN), which itself builds on IEEE 802.15.4 to offer IP-based networking. Internet Protocol (IP) provides the core mechanism for relaying datagrams across IP networks, and its routing capabilities enable internetworking.
+
+#### Mbed Wi-SUN API
+
+Mbed OS provides the [Mesh C++ API](../apis/mesh-api.html) for building Wi-SUN applications.
+
+- To connect to the Wi-SUN network, use the [Wi-SUN interface API](https://github.com/ARMmbed/mbed-os/blob/master/features/nanostack/mbed-mesh-api/mbed-mesh-api/WisunInterface.h).
+- For the socket communication over the Wi-SUN network, use the [Mbed sockets API](../apis/network-socket.html).
+
+##### Nanostack Wi-SUN API
+Mbed Wi-SUN is implemented in the Nanostack library that provides a set of C API headers with more functionalities. The [nanostack folder](https://github.com/ARMmbed/mbed-os/tree/master/features/nanostack/sal-stack-nanostack/nanostack) has the following header files that can be used with Wi-SUN:
+- `net_interface.h` for initializing, starting and stopping a Wi-SUN interface.
+- `ws_management_api.h` for configuring device behaviour.
+- `ws_bbr_api.h` for Wi-SUN backbone border router application interface.
+
+#### Mbed Wi-SUN examples
+Mbed Wi-SUN network consists of routers and border router(s). Mbed OS provides example applications to build Wi-SUN application.
+
+##### Router
+
+- An example using the [Mesh C++ API](../apis/mesh-api.html) for building a Wi-SUN router device can be found from [mesh minimal example](https://github.com/ARMmbed/mbed-os-example-mesh-minimal).
+
+#####Border router
+
+- An example using the [Nanostack API](https://github.com/ARMmbed/mbed-os/tree/master/features/nanostack/sal-stack-nanostack/nanostack) for building a Wi-SUN border router can be found from [nanostack border router](https://github.com/ARMmbed/nanostack-border-router).