document that env and dyn provide the Mapping API

Author: Technologicat

README.md

@@ -245,6 +245,8 @@ print(e)  # empty!
 
 When the `with` block exits, the environment clears itself. The environment instance itself remains alive due to Python's scoping rules.
 
+*Changed in v0.13.1.* ``env`` now provides the ``collections.abc.Mapping`` API.
+
 
 ### ``assignonce``
 
@@ -319,6 +321,8 @@ For some more details, see [the unit tests](unpythonic/test/test_dynassign.py).
 
 *Changed in v0.13.0.* The ``asdict`` and ``items`` methods previously returned a snapshot; now they return a live view.
 
+*Changed in v0.13.1.* ``dyn`` now provides the ``collections.abc.Mapping`` API.
+
 
 ## Containers