add contribution docs

Author: najuzilu

README.md

@@ -33,6 +33,11 @@ extensions = ["sphinxcontrib.prettyproof"]
 See the [Sphinxcontrib Pretty Proof documentation](https://sphinxcontrib-prettyproof.readthedocs.io/en/latest/) for more information.
 
 
+## Contributing
+
+We welcome all contributions! See the [EBP Contributing Guide](https://executablebooks.org/en/latest/contributing.html) for general details, and below for guidance specific to sphinxcontrib-prettyproof.
+
+
 [rtd-badge]: https://readthedocs.org/projects/sphinxcontrib-prettyproof/badge/?version=latest
 [rtd-link]: https://sphinxcontrib-prettyproof.readthedocs.io/en/latest/?badge=latest
 [github-ci]: https://github.com/najuzilu/sphinxcontrib-prettyproof/workflows/continuous-integration/badge.svg?branch=master