Update readme with RTD badge

Author: sirosen

README.md

@@ -2,6 +2,7 @@
 [![supported pythons](https://img.shields.io/pypi/pyversions/check-jsonschema.svg)](https://pypi.org/project/check-jsonschema/)
 [![build](https://github.com/python-jsonschema/check-jsonschema/actions/workflows/build.yaml/badge.svg)](https://github.com/python-jsonschema/check-jsonschema/actions/workflows/build.yaml)
 [![pre-commit.ci status](https://results.pre-commit.ci/badge/github/python-jsonschema/check-jsonschema/main.svg)](https://results.pre-commit.ci/latest/github/python-jsonschema/check-jsonschema/main)
+[![readthedocs documentation](https://readthedocs.org/projects/check-jsonschema/badge/?version=stable&style=flat)](https://check-jsonschema.readthedocs.io/en/stable)
 
 
 # check-jsonschema