use global contributing guide (#484)

Author: davidism

CONTRIBUTING.rst

@@ -38,3 +38,11 @@ users, and allow the maintainers to devote more time to the projects,
 [please donate today][].
 
 [please donate today]: https://palletsprojects.com/donate
+
+## Contributing
+
+See our [detailed contributing documentation][contrib] for many ways to
+contribute, including reporting issues, requesting features, asking or answering
+questions, and making PRs.
+
+[contrib]: https://palletsprojects.com/contributing/