chore: add build status

README.md

-# vue-loader@next
+# vue-loader@next [![Build Status](https://circleci.com/gh/vuejs/vue-loader/tree/next.svg?style=shield)](https://circleci.com/gh/vuejs/vue-loader/tree/next) [![Windows Build status](https://ci.appveyor.com/api/projects/status/8cdonrkbg6m4k1tm/branch/next?svg=true)](https://ci.appveyor.com/project/yyx990803/vue-loader/branch/next)
 
 This is the WIP branch of the next version of `vue-loader`. It uses a fairly different new architecture that is able to apply whatever rules defined in the main webpack config to the language blocks inside a `*.vue` file.