add placeholder and explanation inviting local patches to isolinux.cfg

this closes grml/grml-live #45

This is unfortunately completely untested, since to multiple issues, one
of them being grml/grml-live #44 and grml/grml-live #46 are still
unaddressed, I cannot currently reliably build grml images. As it is a
comment-only patch, this can be safely applied and I'll happily test a
daily image afterwards.