Add coverage badge into README.md

Author: PeachScript

README.md

@@ -1,6 +1,7 @@
 <p align="center"><img width="200" src="./doc/logo.png"></p>
 <p align="center">
   <a href="https://travis-ci.org/PeachScript/vue-infinite-loading"><img src="https://img.shields.io/travis/PeachScript/vue-infinite-loading/master.svg"></a>
+  <a href="https://codecov.io/github/PeachScript/vue-infinite-loading?branch=master"><img src="https://img.shields.io/codecov/c/github/PeachScript/vue-infinite-loading/master.svg"></a>
   <a href="https://www.npmjs.com/package/vue-infinite-loading"><img src="https://img.shields.io/npm/v/vue-infinite-loading.svg"></a>
   <a href="https://www.npmjs.com/package/vue-infinite-loading"><img src="https://img.shields.io/npm/l/vue-infinite-loading.svg"></a>
   <a href="https://david-dm.org/PeachScript/vue-infinite-loading?type=dev"><img src="https://img.shields.io/david/dev/PeachScript/vue-infinite-loading.svg"></a>