Docs(badges): Added coverage status

[ci skip]

Author: fvdm

README.md

@@ -5,6 +5,7 @@ requestbin
 
 [![npm](https://img.shields.io/npm/v/requestbin.svg?maxAge=3600)](https://github.com/fvdm/nodejs-requestbin/blob/master/CHANGELOG.md)
 [![Build Status](https://travis-ci.org/fvdm/nodejs-requestbin.svg?branch=master)](https://travis-ci.org/fvdm/nodejs-requestbin)
+[![Coverage Status](https://coveralls.io/repos/github/fvdm/nodejs-requestbin/badge.svg?branch=develop)](https://coveralls.io/github/fvdm/nodejs-requestbin?branch=develop)
 [![Dependencies](https://www.bithound.io/github/fvdm/nodejs-requestbin/badges/dependencies.svg)](https://www.bithound.io/github/fvdm/nodejs-requestbin/develop/dependencies/npm)
 [![Code Quality](https://www.bithound.io/github/fvdm/nodejs-requestbin/badges/code.svg)](https://www.bithound.io/github/fvdm/nodejs-requestbin)