Add coverage badge

Author: Irfan Maulana

README.md

@@ -1,6 +1,6 @@
 # VueRangedatePicker
 
-[![npm](https://img.shields.io/npm/v/vue-rangedate-picker.svg)](https://www.npmjs.com/package/vue-rangedate-picker) [![vue2](https://img.shields.io/badge/vue-2.x-brightgreen.svg)](https://vuejs.org/) [![Travis](https://img.shields.io/travis/bliblidotcom/vue-rangedate-picker.svg)](https://travis-ci.org/bliblidotcom/vue-rangedate-picker)
+[![npm](https://img.shields.io/npm/v/vue-rangedate-picker.svg)](https://www.npmjs.com/package/vue-rangedate-picker) [![vue2](https://img.shields.io/badge/vue-2.x-brightgreen.svg)](https://vuejs.org/) [![travis](https://img.shields.io/travis/bliblidotcom/vue-rangedate-picker.svg)](https://travis-ci.org/bliblidotcom/vue-rangedate-picker) [![codecov](https://codecov.io/gh/bliblidotcom/vue-rangedate-picker/branch/master/graph/badge.svg)](https://codecov.io/gh/bliblidotcom/vue-rangedate-picker)
 
 > Date picker with range selection