docs(README): add circleci badge

Author: Weffe

README.md

@@ -2,6 +2,7 @@
 
 [![npm version](http://img.shields.io/npm/v/axios-api-versioning.svg)](https://npmjs.org/package/axios-api-versioning "View this project on npm")
 [![Greenkeeper badge](https://badges.greenkeeper.io/Weffe/axios-api-versioning.svg)](https://greenkeeper.io/)
+[![CircleCI](https://circleci.com/gh/Weffe/axios-api-versioning/tree/master.svg?style=svg)](https://circleci.com/gh/Weffe/axios-api-versioning/tree/master)
 
 Add easy to manage api versioning to [axios](https://github.com/axios/axios)