Add cdnjs to the README

This closes #2638.

README.md

@@ -27,7 +27,7 @@ Browser compatibility
 
 Usage
 -----
-You can source Select2 directly from a [CDN like JSDliver](http://www.jsdelivr.com/#!select2), [download it from this GitHub repo](https://github.com/ivaynberg/select2/tags), or use one of the integrations below.
+You can source Select2 directly from a CDN like [JSDliver](http://www.jsdelivr.com/#!select2) or [CDNJS](http://www.cdnjs.com/libraries/select2), [download it from this GitHub repo](https://github.com/ivaynberg/select2/tags), or use one of the integrations below.
 
 Integrations
 ------------