Add jsDelivr hits badge (#2192)

README.md

@@ -5,6 +5,7 @@
 [![license](http://img.shields.io/badge/license-MIT-brightgreen.svg?style=flat)](https://github.com/c3js/c3/blob/master/LICENSE)
 [![codecov.io](https://codecov.io/github/c3js/c3/coverage.svg?branch=master)](https://codecov.io/github/c3js/c3?branch=master)
 [![Greenkeeper badge](https://badges.greenkeeper.io/c3js/c3.svg)](https://greenkeeper.io/)
+[![jsDelivr Hits](https://data.jsdelivr.com/v1/package/npm/c3/badge?style=rounded)](https://www.jsdelivr.com/package/npm/c3)
 
 > c3 is a D3-based reusable chart library that enables deeper integration of charts into web applications.