data/gnome/README: include note on z-ordering
data/gnome/README: include note on z-ordering

As discussed in https://github.com/libratbag/piper/issues/48

libratbag/libratbag,libratbag/libratbag,whot/libratbag,libratbag/libratbag,whot/libratbag,whot/libratbag