docs: add link to 3.x demo

Author: javier-godoy

README.md

@@ -17,8 +17,9 @@ Vaadin 14+ Java integration of [xterm.js](https://xtermjs.org/) terminal emulato
 
 ## Online demo
 
-* [Vaadin 14](http://addonsv14.flowingcode.com/xterm) (Addon version 1.x)
-* [Vaadin 23](http://addonsv23.flowingcode.com/xterm) (Addon version 2.x)
+* [Vaadin 14](http://addonsv14.flowingcode.com/xterm) (Add-on version 1.x)
+* [Vaadin 23](http://addonsv23.flowingcode.com/xterm) (Add-on version 2.x)
+* [Vaadin 24](https://addonsv24.flowingcode.com/xterm) (Add-on version 3.x)
 
 ## Download release