The way I see things:
robots.txtcontains information and instructions for robots (so it should be read/used by web crawlers, spiders and other kind of bots)humans.txtcontains useful information to be consumed by humans, according to http://humanstxt.org/hackers.txtshould be targeted towards hackers, so it should contain any information the site owner might want to transmit to a hacker, as Ze’ev pointed out. I don’t think this should be a place for hackers to write anything, but rather to get information from the site owner (perhaps on how to report vulnerabilities, as others suggested).