Adds gitter label

Author: sobolevn

README.md

@@ -4,6 +4,7 @@
 
 [![Build Status](https://travis-ci.org/typeddjango/pytest-mypy-plugins.svg?branch=master)](https://travis-ci.org/typeddjango/pytest-mypy-plugins)
 [![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)
+[![Gitter](https://badges.gitter.im/mypy-django/Lobby.svg)](https://gitter.im/mypy-django/Lobby)
 
 
 ## Installation