obsinfo is undocumented

[aspiers]

With all the other work I've had in the last few months, I can't even remember what the obsinfo stuff is, let alone how it works. This makes it really hard to review stuff like #138. Please can you help my aching brain by writing one or two short paragraphs which explain it? I think they should probably go in the README.md.

I admit that in general tar_scm could do with better documentation, but at least the README.md makes a half-hearted attempt to explain the standard functionality. Unfortunately it got left behind when adding new functionality.