Add Badge for Artifact Hub (#15958)

We just published our Helm Chart on Artifact Hub: https://artifacthub.io/packages/helm/apache-airflow/airflow

This commit adds a nice badge for Readme

README.md

@@ -27,6 +27,7 @@
 [![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/)
+[![Artifact HUB](https://img.shields.io/endpoint?url=https://artifacthub.io/badge/repository/apache-airflow)](https://artifacthub.io/packages/search?repo=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)