Add reference to development methodology.

Ref python/cpython#123144

importlib_resources/__init__.py

@@ -1,4 +1,11 @@
-"""Read resources contained within a package."""
+"""
+Read resources contained within a package.
+
+This codebase is shared between importlib.resources in the stdlib
+and importlib_resources in PyPI. See
+https://github.com/python/importlib_metadata/wiki/Development-Methodology
+for more detail.
+"""
 
 from ._common import (
     as_file,