Add a Gitter chat badge to README.md (#35)

* Add Gitter badge

* Update README.md

Author: gitter-badger

README.md

@@ -3,6 +3,7 @@
 [![npm version](https://badge.fury.io/js/github-release-notes.svg)](https://badge.fury.io/js/github-release-notes)
 [![js-standard-style](https://img.shields.io/badge/code%20style-standard-brightgreen.svg)](http://standardjs.com)
 [![Build Status](https://travis-ci.org/github-tools/github-release-notes.svg?branch=master)](https://travis-ci.org/github-tools/github-release-notes)
+[![Join the chat at https://gitter.im/github-release-notes/Lobby](https://badges.gitter.im/github-release-notes/Lobby.svg)](https://gitter.im/github-release-notes/Lobby?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 > Node module that generates release notes based on commit messages or closed issues between tags. It also can create a full changelog or add the latest release notes to the existing changelog file.