Chapter Prose

First draft of the chapter prose, in rst form for now. I'm requesting PR to have someone look at the prose. I've also included an svg diagram, but you'll have to pull the repository to see it. I don't think it's viewable within github...

If you want to pull this, this is the 'chapter' branch from https://github.com/malini/500lines
There is a relative large amount of text in this section on how to use the test runner, but I think it might be more important to explain things like why the dispatcher will be pinging (though that does receive two sentences of explanation), or a deeper explanation of the lifetime of test runner. I think more people will be reading the code than running it and would benefit more from a more thorough explanation.