Add Coveralls Badge

Author: ktsujichan

README.md

@@ -4,6 +4,7 @@
 [![Build Status](https://travis-ci.org/ktsujichan/qiita-sdk-go.svg?branch=master)](https://travis-ci.org/ktsujichan/qiita-sdk-go)
 [![Code Climate](https://codeclimate.com/github/ktsujichan/qiita-sdk-go/badges/gpa.svg)](https://codeclimate.com/github/ktsujichan/qiita-sdk-go)
 [![Issue Count](https://codeclimate.com/github/ktsujichan/qiita-sdk-go/badges/issue_count.svg)](https://codeclimate.com/github/ktsujichan/qiita-sdk-go)
+[![Coverage Status](https://coveralls.io/repos/github/ktsujichan/qiita-sdk-go/badge.svg?branch=master)](https://coveralls.io/github/ktsujichan/qiita-sdk-go?branch=master)
 [![contributions welcome](https://img.shields.io/badge/contributions-welcome-brightgreen.svg?style=flat)](https://github.com/ktsujichan/qiita-sdk-go/issues)
 </span>