Add gitter badge

Author: sagikazarmark

README.md

@@ -4,6 +4,8 @@
 [![Software License](https://img.shields.io/badge/license-MIT-brightgreen.svg?style=flat-square)](LICENSE)
 [![Total Downloads](https://img.shields.io/packagist/dt/php-http/adapter.svg?style=flat-square)](https://packagist.org/packages/php-http/adapter)
 
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/php-http/adapter?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 **HTTP Adapter interfaces.**