Add note in CONTRIBUTING

CONTRIBUTING.md

@@ -26,6 +26,16 @@ click around. You can refresh your browser after each `make coverage` run.
 
 Formatting is done with `black`. Run `make fmt`.
 
-Docstrings follow the [Google style][docstrings].
+Docstrings follow the [Google style][docstrings]. All public entities should
+have a docstring attached.
 
 [docstrings]: https://google.github.io/styleguide/pyguide.html#s3.8-comments-and-docstrings
+
+## Documentation
+
+API reference documentation is automatically built on merges to `main` and new
+tagged version. To view the generated documentation locally:
+
+```
+make local-docs
+```