Update CHANGES.md for 0.6.1.
Update CHANGES.md for 0.6.1.

ocamllabs/ocaml-ctypes,yallop/ocaml-ctypes,dsheets/ocaml-ctypes,dsheets/ocaml-ctypes,whitequark/ocaml-ctypes,whitequark/ocaml-ctypes,dsheets/ocaml-ctypes,ocamllabs/ocaml-ctypes,whitequark/ocaml-ctypes,yallop/ocaml-ctypes