Add compatibility matrix comparing importlib_metadata versions and th…

…e associated stdlib version. Fixes #306.

CHANGES.rst

@@ -1,3 +1,8 @@
+v4.0.1
+=======
+
+* #306: Clearer guidance about compatibility in readme.
+
 v4.0.0
 =======
 

README.rst

@@ -20,11 +20,27 @@
 
 Library to access the metadata for a Python package.
 
-As of Python 3.8, this functionality has been added to the
-`Python standard library
-<https://docs.python.org/3/library/importlib.metadata.html>`_.
-This package supplies backports of that functionality including
-improvements added to subsequent Python versions.
+This package supplies third-party access to the functionality of
+`importlib.metadata <https://docs.python.org/3/library/importlib.metadata.html>`_
+including improvements added to subsequent Python versions.
+
+
+Compatibility
+=============
+
+New features are introduced in this third-party library and later merged
+into CPython. The following table indicates which versions of this library
+were contributed to different versions in the standard library:
+
+.. list-table::
+   :header-rows: 1
+
+   * - importlib_metadata
+     - stdlib
+   * - 3.10
+     - 3.10
+   * - 1.4
+     - 3.8
 
 
 Usage