I'd say, yes, put a donation button on there. And be sure to mention it at the end of every tutorial. "If you liked the tutorial, consider giving a donation to support ... etc etc..."

Done the right way people will not be offended by it. Make sure to put up a special page detailing what you just said in this blog entry too. I'd consider donating a few euro's whenever something is usefull to me and it's free.