Add Badges for Black Code-Style & PyPI Downloads (apache#12076)

Airflow gets around 513k downloads per month on PyPI and we just merged PR to enable black across our codebase

README.md

@@ -26,7 +26,8 @@
 [![PyPI - Python Version](https://img.shields.io/pypi/pyversions/apache-airflow.svg)](https://pypi.org/project/apache-airflow/)
 [![Docker Pulls](https://img.shields.io/docker/pulls/apache/airflow.svg)](https://hub.docker.com/r/apache/airflow)
 [![Docker Stars](https://img.shields.io/docker/stars/apache/airflow.svg)](https://hub.docker.com/r/apache/airflow)
-
+[![PyPI - Downloads](https://img.shields.io/pypi/dm/apache-airflow)](https://pypi.org/project/apache-airflow/)
+[![Code style: black](https://img.shields.io/badge/code%20style-black-000000.svg)](https://github.com/psf/black)
 [![Twitter Follow](https://img.shields.io/twitter/follow/ApacheAirflow.svg?style=social&label=Follow)](https://twitter.com/ApacheAirflow)
 [![Slack Status](https://img.shields.io/badge/slack-join_chat-white.svg?logo=slack&style=social)](https://s.apache.org/airflow-slack)