docs: add badge to readme

Author: ianhi

README.md

@@ -5,5 +5,6 @@
 [![Python Version](https://img.shields.io/pypi/pyversions/mpl-image-segmenter.svg?color=green)](https://python.org)
 [![CI](https://github.com/ianhi/mpl-image-segmenter/actions/workflows/ci.yml/badge.svg)](https://github.com/ianhi/mpl-image-segmenter/actions/workflows/ci.yml)
 [![codecov](https://codecov.io/gh/ianhi/mpl-image-segmenter/branch/main/graph/badge.svg)](https://codecov.io/gh/ianhi/mpl-image-segmenter)
+[![Documentation Status](https://readthedocs.org/projects/mpl-image-segmenter/badge/?version=stable)](https://mpl-image-segmenter.readthedocs.io/en/latest/?badge=latest)
 
 Manually segment images with matplotlib