Mention mandatory code review policy
diff --git a/CONTRIBUTING b/CONTRIBUTING
index 7b03ba0..3e47ec6 100644
--- a/CONTRIBUTING
+++ b/CONTRIBUTING
@@ -1,3 +1,6 @@
+This repository has a mandatory code review policy. Contributions
+should happen through pull requests. Never commit to master directly.
+
 When in doubt, refer to PEP 8 for Python code.
 
 Use PEP 257 for docstring conventions, except use Sphinx notation for