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
I think it would be helpful to specifically mention the separation/isolation aspect here. The test runner operates separately, and it's only a coincidence that it's running on the same host as the other components. It will need its own repository so it can be portable and scalable (i.e., so you can run test runners on separate hosts) and so it's isolated from the other components, should you choose to run them on the same host. This is a key point of distributed systems: they span multiple hosts and have defined methods of communication, or can be run on a single host for testing.