Added release notes about Python changes.
diff --git a/CHANGES.txt b/CHANGES.txt
index e0c0208..3aa0533 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -1,8 +1,25 @@
2015-02-22 version 3.0.0-alpha-2 (Ruby/JavaNano):
General
* Introduced two new language implementations (Ruby and JavaNano) to proto3.
+ * Added proto3 support for Python, and various other improvements.
* Various bug fixes since 3.0.0-alpha-1
+ Python:
+ Python has received several updates, most notably support for proto3
+ semantics in any .proto file that declares syntax="proto3".
+ Messages declared in proto3 files no longer represent field presence
+ for scalar fields (number, enums, booleans, or strings). You can
+ no longer call HasField() for such fields, and they are serialized
+ based on whether they have a non-zero/empty/false value.
+
+ One other notable change is in the C++-accelerated implementation.
+ Descriptor objects (which describe the protobuf schema and allow
+ reflection over it) are no longer duplicated between the Python
+ and C++ layers. The Python descriptors are now simple wrappers
+ around the C++ descriptors. This change should significantly
+ reduce the memory usage of programs that use a lot of message
+ types.
+
Ruby:
We have added proto3 support for Ruby via a native C extension.
