Document the helper interface from the helper PoV
    
    The submit.txt document is written from the daemon's point of view, and
    while that's great for maintainers who want a refresher on what the
    daemon's supposed to be doing, it's not very helpful for people who want
    to write enrollment helpers.
    
    This document is meant to ultimately replace submit.txt as the
    authoritative document -- if keeping this file and submit.txt in sync
    becomes burdensome, we'll drop submit.txt.