add coverage badge

README.md

@@ -1,4 +1,4 @@
-# vue-next [![CircleCI](https://img.shields.io/circleci/project/vuejs/vue/next.svg?maxAge=2592000)](https://circleci.com/gh/vuejs/vue/tree/next)
+# vue-next [![CircleCI](https://img.shields.io/circleci/project/vuejs/vue/next.svg?maxAge=2592000)](https://circleci.com/gh/vuejs/vue/tree/next) [![Coverage](https://img.shields.io/codecov/c/github/vuejs/vue/next.svg)](https://codecov.io/github/vuejs/vue?branch=next)
 
 > pre-alpha state, use at your own risk. For the time being, check out the [announcement post](https://medium.com/the-vue-point/announcing-vue-js-2-0-8af1bde7ab9#.jylu9wz4y) and the [feature comparison with 1.0](https://github.com/vuejs/vue/wiki/2.0-features).