update README

Author: erich2s

README.md

@@ -3,6 +3,8 @@
   <p>React-Mqtt-Hooks</p>
 </h1>
 
+See demo here: <https://react-mqtt-hooks.vercel.app/>
+
 ## Introduction
 
 React-Mqtt-Hooks is a library that simplifies the integration of [MQTT](https://mqtt.org/) (Message Queuing Telemetry Transport) functionality into React applications. It provides a set of custom hooks that allow developers to easily connect to an MQTT broker based on the popular [MQTT.js](https://github.com/mqttjs/MQTT.js) library.