Expand docs on send_file option hook, #433.

CHANGES

@@ -48,6 +48,10 @@ Relase date to be decided, codename to be chosen.
 - View functions can now return a tuple with the first instance being an
   instance of :class:`flask.Response`.  This allows for returning
   ``jsonify(error="error msg"), 400`` from a view function.
+- :class:`flask.Flask` now provides a `get_static_file_options` hook for
+  subclasses to override behavior of serving static files through Flask,
+  optionally by filename, which for example allows changing cache controls by
+  file extension.
 
 
 Version 0.8.1