Reformat CHANGES.md
Reformat CHANGES.md

It is now topkg-friendly.

Signed-off-by: David Scott <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@recoil.org>

djs55/ocaml-vchan,mirage/ocaml-vchan,mirage/ocaml-vchan,djs55/ocaml-vchan,djs55/ocaml-vchan,mirage/ocaml-vchan