add MDN WebSocket API reference

Author: Wasin Waeosri

README.md

@@ -60,5 +60,6 @@ For further details, please check out the following resources:
 * [Developer Webinar Recording: Introduction to Electron Websocket API](https://www.youtube.com/watch?v=CDKWMsIQfaw)
 * [Mozilla Developer Network: Web Workers API page](https://developer.mozilla.org/en-US/docs/Web/API/Web_Workers_API).
 * [Google HTML5 Rocks: The Basics of Web Workers page](https://www.html5rocks.com/en/tutorials/workers/basics/)
+* [Mozilla Developer Network: WebSocket API page](https://developer.mozilla.org/en-US/docs/Web/API/WebSockets_API)
 
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