docs: add badge [skip ci]

Author: kt3k

README.md

@@ -1,5 +1,6 @@
 # [nodejs.jp][homepage]
 
+[![CircleCI](https://circleci.com/gh/nodejsjp/nodejsjp.github.com.svg?style=svg)](https://circleci.com/gh/nodejsjp/nodejsjp.github.com)
 [![](https://img.shields.io/badge/SLACK-JOIN_CHAT_%E2%86%92-551a8b.svg?style=flat-square)](https://iojs-jp-slack.herokuapp.com/)
 
 # インストール