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
It seems as though there is a lot within this section on "Limitations and Notes" and relatively little in the next section, the introduction. I think some more time could be spent in the introduction explaining the "why" of the system, since much of the chapter is spent on the "how". Why were key decisions made, what are the alternatives, why would some alternatives be better or worse - that sort of thing.