demo with gif (#3995)

add demo.gif to README.md

Author: khassel

README.md

@@ -15,6 +15,8 @@
 
 MagicMirror² focuses on a modular plugin system and uses [Electron](https://www.electronjs.org/) as an application wrapper. So no more web server or browser installs necessary!
 
+![](https://magicmirror.builders/img/demo.gif)
+
 ## Documentation
 
 For the full documentation including **[installation instructions](https://docs.magicmirror.builders/getting-started/installation.html)**, please visit our dedicated documentation website: [https://docs.magicmirror.builders](https://docs.magicmirror.builders).