Obnam 3: documentation; open up project

Goal I want to open up the project for contributions from others. I think the functionality is now there: there's a bunch of things I'd like to still do, but it might be better to not polish too much. Instead, I'll want to make it easy to start hacking on the code. My goal today is to review all documentation and update it as needed. I'll also want to make sure what is useful to publish is…