Add readthedocs build badge

README.md

@@ -1,4 +1,6 @@
 [![Build Status](https://github.com/imagingdatacommons/highdicom/actions/workflows/run_unit_tests.yml/badge.svg)](https://github.com/imagingdatacommons/highdicom/actions)
+[![Build Status](https://github.com/imagingdatacommons/highdicom/actions/workflows/run_unit_tests.yml/badge.svg)](https://github.com/imagingdatacommons/highdicom/actions)
+[![Documentation Status](https://readthedocs.org/projects/highdicom/badge/?version=latest)](https://highdicom.readthedocs.io/en/latest/?badge=latest)
 [![PyPi Distribution](https://img.shields.io/pypi/v/highdicom.svg)](https://pypi.python.org/pypi/highdicom/)
 [![Python Versions](https://img.shields.io/pypi/pyversions/highdicom.svg)](https://pypi.org/project/highdicom/)
 [![Downloads](https://pepy.tech/badge/highdicom)](https://pepy.tech/project/highdicom)