Add reference to development methodology.

Ref python/cpython#123144

Author: jaraco

importlib_metadata/__init__.py

@@ -1,3 +1,12 @@
+"""
+APIs exposing metadata from third-party Python packages.
+
+This codebase is shared between importlib.metadata in the stdlib
+and importlib_metadata in PyPI. See
+https://github.com/python/importlib_metadata/wiki/Development-Methodology
+for more detail.
+"""
+
 from __future__ import annotations
 
 import abc