use global contributing guide

Remove the per-project files so we don't have to keep them in sync.
GitHub's UI links to everything except the contributing guide, so add a
section about that to the readme.

Author: davidism

CONTRIBUTING.rst

@@ -47,3 +47,11 @@ 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/