Merge pull request nuxt#2784 from qm3ster/readme-circleci-badge

readme: Add circleci badge

Author: clarkdo

README.md

@@ -1,5 +1,6 @@
 <p align="center"><img align="center" src="http://imgur.com/V4LtoII.png"/></p>
 <p align="center">
+  <a href="https://circleci.com/gh/nuxt/nuxt.js"><img src="https://img.shields.io/circleci/project/github/nuxt/nuxt.js.svg" alt="Build Status"></a>
   <a href="https://travis-ci.org/nuxt/nuxt.js"><img src="https://img.shields.io/travis/nuxt/nuxt.js/master.svg" alt="Build Status"></a>
   <a href="https://ci.appveyor.com/project/Atinux/nuxt-js"><img src="https://ci.appveyor.com/api/projects/status/gwab06obc6srx9g4?svg=true" alt="Windows Build Status"></a>
   <a href="https://codecov.io/gh/nuxt/nuxt.js"><img src="https://img.shields.io/codecov/c/github/nuxt/nuxt.js/master.svg" alt="Coverage Status"></a>