Added Changelog entries for Google-internal changes.

CHANGES.txt

@@ -1,3 +1,13 @@
+Unreleased Changes
+
+  C++:
+  * Enabled heterogeneous lookup for std::string keys in maps.
+  * Improved the randomness of map ordering.
+
+  Python:
+  * Improved the error message when AttributeError is returned from __getattr__
+    in EnumTypeWrapper.
+
 2020-05-12 version 3.12.0 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
 
   Protocol Compiler