Add python Gitter channel badge

Co-authored-by: Rob Emanuele <rdemanuele@gmail.com>

Author: duckontheweb

README.md

@@ -3,7 +3,7 @@
 [![PyPI version](https://badge.fury.io/py/pystac.svg)](https://badge.fury.io/py/pystac)
 [![Documentation](https://readthedocs.org/projects/pystac/badge/?version=latest)](https://pystac.readthedocs.io/en/latest/)
 [![codecov](https://codecov.io/gh/stac-utils/pystac/branch/main/graph/badge.svg)](https://codecov.io/gh/stac-utils/pystac)
-
+[![Gitter](https://badges.gitter.im/SpatioTemporal-Asset-Catalog/python.svg)](https://gitter.im/SpatioTemporal-Asset-Catalog/python?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0)
 
 PySTAC is a library for working with [SpatialTemporal Asset Catalog](https://stacspec.org) in Python 3.