document a virtual "sop" interface for debian

@guillem suggested on IRC that any sop implementation that makes it into debian should be able to use the update-alternatives mechanism to provide a system sop. He also suggests that maybe a virtual Provides: (maybe versioned by the draft number?) would be useful.

It would be great to do that for at least one sop implementation, and to document how to do it in the repo here (though it's probably too system-specific to put it in the Internet Draft directly)