Add a mypy badge to README (#130)

Author: ilevkivskyi

README.md

@@ -2,7 +2,7 @@
 
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-[![Build Status](https://travis-ci.org/dry-python/returns.svg?branch=master)](https://travis-ci.org/dry-python/returns) [![Coverage Status](https://coveralls.io/repos/github/dry-python/returns/badge.svg?branch=master)](https://coveralls.io/github/dry-python/returns?branch=master) [![Documentation Status](https://readthedocs.org/projects/returns/badge/?version=latest)](https://returns.readthedocs.io/en/latest/?badge=latest) [![Python Version](https://img.shields.io/pypi/pyversions/returns.svg)](https://pypi.org/project/returns/) [![wemake-python-styleguide](https://img.shields.io/badge/style-wemake-000000.svg)](https://github.com/wemake-services/wemake-python-styleguide)
+[![Build Status](https://travis-ci.org/dry-python/returns.svg?branch=master)](https://travis-ci.org/dry-python/returns) [![Coverage Status](https://coveralls.io/repos/github/dry-python/returns/badge.svg?branch=master)](https://coveralls.io/github/dry-python/returns?branch=master) [![Documentation Status](https://readthedocs.org/projects/returns/badge/?version=latest)](https://returns.readthedocs.io/en/latest/?badge=latest) [![Python Version](https://img.shields.io/pypi/pyversions/returns.svg)](https://pypi.org/project/returns/) [![wemake-python-styleguide](https://img.shields.io/badge/style-wemake-000000.svg)](https://github.com/wemake-services/wemake-python-styleguide) [![Checked with mypy](http://www.mypy-lang.org/static/mypy_badge.svg)](http://mypy-lang.org/)
 
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