I really love this idea, a standard place to find the authors and the tools (!) used. But I'm not a big fan of the name. Calling it humans.txt to mirror robots.txt doesn't make much sense to me, even as a joke. I think it should be named something direct and comprehensible like credits.txt.
Very cool, I've never heard of this before but really like the idea. Kind of a tech secret to find out who's behind a site.