Merge pull request #693 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md

README.md

 # tarantool [![Build Status](https://travis-ci.org/tarantool/tarantool.png?branch=master)](https://travis-ci.org/tarantool/tarantool)
 
+[![Gitter](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/tarantool/tarantool?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+
 http://tarantool.org
 
 Tarantool is an efficient NoSQL database and a