diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 994698cca..4e7952399 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -211,7 +211,9 @@ Some further tips for good type hints:
   positions, in favor of covariant types like `Mapping` or `Sequence`;
 * avoid Union return types: https://github.com/python/mypy/issues/1693;
 * in Python 2, whenever possible, use `unicode` if that's the only
-  possible type, and `Text` if it can be either `unicode` or `bytes`.
+  possible type, and `Text` if it can be either `unicode` or `bytes`;
+* use platform checks like `if sys.platform == 'win32'` to denote
+  platform-dependent APIs.
 
 Imports in stubs are considered private (not part of the exported API)
 unless: