CHANGELOG, add important notes: DUNE-multidomain(grid)

doesn't work with DUNE 2.4 at the moment,
therefore DuMuX multidomain models only work
with DUNE 2.3 at the moment


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@14508 2fb0f335-1f38-0410-981e-8018bf24f1b0