add black codestyle badge to README (#829)

README.md

@@ -4,6 +4,7 @@
 [![Downloads Per Week](https://pepy.tech/badge/popper/week)](https://pepy.tech/project/popper)
 [![Build Status](https://travis-ci.org/systemslab/popper.svg?branch=master)](https://travis-ci.org/systemslab/popper)
 [![codecov](https://codecov.io/gh/systemslab/popper/branch/master/graph/badge.svg)](https://codecov.io/gh/systemslab/popper)
+[![black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/ambv/black)
 [![PyPI version](https://badge.fury.io/py/popper.svg)](https://badge.fury.io/py/popper)
 [![Join the chat at https://gitter.im/systemslab/popper](https://badges.gitter.im/systemslab/popper.svg)](https://gitter.im/falsifiable-us/popper?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 [![slack](https://img.shields.io/badge/chat-on_slack-C03C20.svg?logo=slack)](https://join.slack.com/t/getpopper/shared_invite/zt-dtn0se2s-c50myMHNpeoikQXDeNbPew)