add section on Python 2 compatibility

pep-0461.txt

@@ -159,6 +159,20 @@ Unsupported codes
 ``%r`` (which calls ``__repr__`` and returns a ``str``) is not supported.
 
 
+Compatibility with Python 2
+===========================
+
+As noted above, ``%s`` is being included solely to help ease migration from,
+and/or have a single code base with, Python 2.  This is important as there
+are modules both in the wild and behind closed doors that currently use the
+Python 2 ``str`` type as a ``bytes`` container, and hence are using ``%s``
+as a bytes interpolator.
+
+However, ``%b`` should be used in new, Python 3 only code, so ``%s`` will
+immediately be deprecated, but not removed until the next major Python
+release.
+
+
 Proposed variations
 ===================