CONTRIBUTING: Note no bytearray promotion

Closes #9001

Author: JelleZijlstra

CONTRIBUTING.md

@@ -286,6 +286,13 @@ Two exceptions are `Protocol` and `runtime_checkable`: although
 these were added in Python 3.8, they can be used in stubs regardless
 of Python version.
 
+[PEP 688](https://www.python.org/dev/peps/pep-0688/), which is
+currently a draft, removes the implicit promotion of the
+`bytearray` and `memoryview` classes to `bytes`.
+Typeshed stubs should be written assuming that this proposal
+is accepted, so a parameter that accepts either `bytes` or
+`bytearray` should be typed as `bytes | bytearray`.
+
 ### What to include
 
 Stubs should include the complete interface (classes, functions,