Add CHANGES entry and versionchanged entry to docstring

CHANGES

@@ -8,6 +8,9 @@ Version 1.0
 
 (release date to be announced, codename to be selected)
 
+- Added `**kwargs` to :meth:`flask.Test.test_client` to support passing
+  additional keyword arguments to the constructor of
+  :attr:`flask.Flask.test_client_class`.
 - Added ``SESSION_REFRESH_EACH_REQUEST`` config key that controls the
   set-cookie behavior.  If set to `True` a permanent session will be
   refreshed each request and get their lifetime extended, if set to

flask/app.py

@@ -867,6 +867,10 @@ class Flask(_PackageBoundObject):
            The `use_cookies` parameter was added as well as the ability
            to override the client to be used by setting the
            :attr:`test_client_class` attribute.
+
+        .. versionchanged:: 1.0
+           Added `**kwargs` to support passing additional keyword arguments to
+           the constructor of :attr:`test_client_class`.
         """
         cls = self.test_client_class
         if cls is None: