Add reference to development methodology.

Ref python/cpython#123144

zipp/__init__.py

@@ -1,3 +1,12 @@
+"""
+A Path-like interface for zipfiles.
+
+This codebase is shared between zipfile.Path in the stdlib
+and zipp in PyPI. See
+https://github.com/python/importlib_metadata/wiki/Development-Methodology
+for more detail.
+"""
+
 import io
 import posixpath
 import zipfile