Mention extension_points in Changes.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14047 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02
PR#6148: speed improvement for Buffer (patch by John Whitington).

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@14048 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02