Add mypy badge to README (#5197)

Author: ilevkivskyi

README.md

@@ -5,6 +5,7 @@ Mypy: Optional Static Typing for Python
 
 [![Build Status](https://api.travis-ci.org/python/mypy.svg?branch=master)](https://travis-ci.org/python/mypy)
 [![Chat at https://gitter.im/python/typing](https://badges.gitter.im/python/typing.svg)](https://gitter.im/python/typing?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)
 
 
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