update changelog to reflect non-bw-incompat change

CHANGES.rst

@@ -1,11 +1,15 @@
 unreleased
 ==========
 
-- We implicitly assign the schema type to MappingSchema, SequenceSchema, and
-  TupleSchema with types of Mapping, Sequence, and Tuple explicitly.  This is
-  a minor backwards incompatibility, because we no longer allow any of those
-  schema variants to accept the implied type as a first argument.
-  i.e. MappingSchema(Mapping()) will no longer work.
+Bug Fixes
+---------
+
+- Fix a bug in which ``MappingSchema``, ``SequenceSchema`` and
+  ``TupleSchema`` would always treat the first arg as the schema type. This
+  meant that it would fail if passed any nodes to the constructor despite
+  the default type being implied by the name. It is now possible to do
+  ``MappingSchema(child1, child2, ...)`` instead of
+  ``MappingSchema(Mapping(), child1, child2)``.
 
 1.1 (2016-01-15)
 ================