A Disclaimer with SmartList

Hello,

I installed a group/list with SmartList and I am very happy,
how easy and good it works. Now, I would like to append
a disclaimer or signature to any mail, send to this list. The text of
this disclaimer will be in disclaimer.txt, and the Manual said,
it's possible to do it with rc.local.s20 - but I don't know
how to do it.
Can anybody send me an example how I can do this?

Thanks a lot for any hint and help.
Volker Dobler