add cdn badges (#5167)

README.md

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 Select2
 =======
 [![Build Status][travis-ci-image]][travis-ci-status]
+[![cdnjs](https://img.shields.io/cdnjs/v/select2.svg)](https://cdnjs.com/libraries/select2)
+[![jsdelivr](https://data.jsdelivr.com/v1/package/npm/select2/badge)](https://www.jsdelivr.com/package/npm/select2)
 
 Select2 is a jQuery-based replacement for select boxes. It supports searching,
 remote data sets, and pagination of results.