Add Charba GWT integration to extensions.md (#5225)

Author: stockiNail

docs/notes/extensions.md

@@ -56,5 +56,8 @@ In addition, many plugins can be found on the [npm registry](https://www.npmjs.c
 ### Java
  - <a href="https://github.com/mdewilde/chart/" target="_blank">Chart.java</a>
 
+### GWT (Google Web toolkit)
+ - <a href="https://github.com/pepstock-org/Charba" target="_blank">Charba</a>
+
 ### Ember.js
  - <a href="https://github.com/aomran/ember-cli-chart" target="_blank">ember-cli-chart</a>