Add CDNJS version badge in README.md

The badge shows the latest lib version on CDNJS and gives a link of it.
chartjs · Oct 23, 2017 · 77f047d · 77f047d
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 # chartjs-plugin-annotation.js
+[![CDNJS version](https://img.shields.io/cdnjs/v/chartjs-plugin-annotation.svg)](https://cdnjs.com/libraries/chartjs-plugin-annotation)
 
 An annotation plugin for Chart.js >= 2.4.0