Update README.md

Author: ypankovych

README.md

@@ -2,6 +2,7 @@
 [![Downloads](https://pepy.tech/badge/pankoff)](https://pepy.tech/project/pankoff)
 [![Documentation Status](https://readthedocs.org/projects/pankoff/badge/?version=12.0)](https://pankoff.readthedocs.io/?badge=12.0)
 [![PyPI pyversions](https://img.shields.io/pypi/pyversions/pankoff.svg)](https://pypi.python.org/pypi/pankoff/)
+[![Maintenance](https://img.shields.io/badge/Maintained%3F-yes-green.svg)](https://GitHub.com/ypankovych/pankoff/graphs/commit-activity)
 
 
 Light weight, flexible, easy to use validation tool. Pure Python, no dependencies.