Add Read the Docs badge

Author: cjolowicz

README.md

@@ -1,6 +1,7 @@
 [![tests](https://github.com/cjolowicz/hypermodern-python/workflows/tests/badge.svg)](https://github.com/cjolowicz/hypermodern-python/actions?workflow=tests)
 [![Codecov](https://codecov.io/gh/cjolowicz/hypermodern-python/branch/master/graph/badge.svg)](https://codecov.io/gh/cjolowicz/hypermodern-python)
 [![PyPI](https://img.shields.io/pypi/v/hypermodern-python.svg)](https://pypi.org/project/hypermodern-python/)
+[![Read the Docs](https://readthedocs.org/projects/hypermodern-python/badge/)](https://hypermodern-python.readthedocs.io/)
 
 # hypermodern-python