add changelog for 0.12.3

CHANGES

@@ -1,26 +1,22 @@
 Flask Changelog
 ===============
 
-Here you can see the full list of changes between each Flask release.
-
-Version 0.13
-------------
-
-Major release, unreleased
-
-- Make `app.run()` into a noop if a Flask application is run from the
-  development server on the command line.  This avoids some behavior that
-  was confusing to debug for newcomers.
-- Change default configuration `JSONIFY_PRETTYPRINT_REGULAR=False`. jsonify()
-  method returns compressed response by default, and pretty response in
-  debug mode.
 
 Version 0.12.3
 --------------
 
 Bugfix release, unreleased
 
-- Fix a ValueError caused by invalid Range requests in some cases
+-   :func:`Request.get_json` no longer accepts arbitrary encodings.
+    Incoming JSON should be encoded using UTF-8 per :rfc:`8259`, but
+    Flask will autodetect UTF-8, -16, or -32. (`#2692`_)
+-   Fix a Python warning about imports when using ``python -m flask``.
+    (`#2666`_)
+-   Fix a ``ValueError`` caused by invalid ``Range`` requests in some
+    cases.
+
+.. _#2666: https://github.com/pallets/flask/issues/2666
+.. _#2692: https://github.com/pallets/flask/issues/2692
 
 
 Version 0.12.2