add initial config docs

[hall]

Opening this PR to start work on defining a schema for the configuration described in #2 as there will likely be a few tools accessing that file. I'd also like to document all the moving parts (insofar as how a user could modify their switch's configuration) as best we can.

I've made some assumptions in the initial draft so please feel free to correct me (and a lot of it is still in flight, of course -- this is more of a document on how it should be once those tools are ready; most of them aren't far off). I'll continue expanding the docs once others have decided it's worthwhile.