Add CDNJS version badge in README.md (#6297)

This will add the badge to show its version on CDNJS and also link to its page on CDNJS!

Author: nolsherry

README.md

@@ -3,6 +3,7 @@
 [![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/angular-ui/bootstrap?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![Build Status](https://secure.travis-ci.org/angular-ui/bootstrap.svg)](http://travis-ci.org/angular-ui/bootstrap)
 [![devDependency Status](https://david-dm.org/angular-ui/bootstrap/dev-status.svg?branch=master)](https://david-dm.org/angular-ui/bootstrap#info=devDependencies)
+[![CDNJS](https://img.shields.io/cdnjs/v/angular-ui-bootstrap.svg)](https://cdnjs.com/libraries/angular-ui-bootstrap/)
 
 ### Quick links
 - [Demo](#demo)