Add coverage badge

Author: brh55

README.md

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-[![Build Status](https://img.shields.io/travis/cisco-ie/stenella.svg?style=flat-square)](https://travis-ci.org/cisco-ie/stenella) [![license](https://img.shields.io/github/license/cisco-ie/stenella.svg?style=flat-square)]() [![XO code style](https://img.shields.io/badge/code_style-XO-5ed9c7.svg?style=flat-square)](https://github.com/sindresorhus/xo)
+[![Build Status](https://img.shields.io/travis/cisco-ie/stenella.svg?style=flat-square)](https://travis-ci.org/cisco-ie/stenella) [![license](https://img.shields.io/github/license/cisco-ie/stenella.svg?style=flat-square)]() [![XO code style](https://img.shields.io/badge/code_style-XO-5ed9c7.svg?style=flat-square)](https://github.com/sindresorhus/xo) [![Coveralls branch](https://img.shields.io/coveralls/cisco-ie/stenella/master.svg?style=flat-square)](https://coveralls.io/github/cisco-ie/stenella)
 
 `stenella` is an unofficial, server-to-server, Node.js application that listens to Google calendars events that are created, deleted, or updated. With each event, developers can define and execute their own business logic in regards to contextual cues found within a calenader event payload *(date, subject, summary, attendees, location, status, etc.)*. These are known as "observers" *(calendar event handlers)*, which are flexible, open-ended, and simple to write.