Add Mongoose overview doc section

PUBLISHED_FROM=9fb2260cd76ae30b51db02053f4d2debf4cdf900

docs/intro.md

@@ -3,12 +3,9 @@ title: Mongoose - a networking library
 color: '#1D6482'
 repo: https://github.com/cesanta/mongoose
 items:
+  - { type: file, name: overview.md }
   - { type: dir, name: usage-example }
   - { type: dir, name: design-concept }
   - { type: dir, name: build-options }
   - { type: flat, name: c-api }
 ---
-
-Mongoose is a multi-protocol embedded networking library with
-that implements TCP, UDP, HTTP, WebSocket, CoAP, MQTT, JSON-RPC,
-both client and server mode.

docs/overview.md

+---
+title: Overview
+---
+
+Mongoose is a swiss army knife for embedded network programming.
+It implements event-driven non-blocking API for TCP, UDP, HTTP,
+WebSocket, CoAP, MQTT, JSON-RPC for both client and server mode.
+Features include:
+
+- Cross-platform: works on Linux/UNIX, MacOS, QNX, eCos, Windows, Android,
+  iPhone, FreeRTOS (TI CC3200, ESP8266), etc
+- Single-threaded, asynchronous, non-blocking core with simple event-based API
+- Native support for [PicoTCP embedded TCP/IP stack](http://www.picotcp.com),
+  [LWIP embedded TCP/IP stack](https://en.wikipedia.org/wiki/LwIP)
+- Builtin protocols:
+   - plain TCP, plain UDP, SSL/TLS (over TCP, one-way or two-way)
+   - HTTP client, HTTP server
+   - WebSocket client, WebSocket server
+   - JSON-RPC client, JSON-RPC server
+   - MQTT client, MQTT broker
+   - CoAP client, CoAP server
+   - DNS client, DNS server, async DNS resolver
+- Tiny static and run-time footprint
+- Source code is both ISO C and ISO C++ compliant
+- Very easy to integrate: just copy
+  [mongoose.c](https://raw.githubusercontent.com/cesanta/mongoose/master/mongoose.c) and
+  [mongoose.h](https://raw.githubusercontent.com/cesanta/mongoose/master/mongoose.h)
+  files to your build tree