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
This paragraph could be clearer with some rewording, especially the last sentence ("can be used for using the filesystem", etc.). Maybe something like:

Alternatively, there are operating system-independent methods for performing these functions, such as the 'os' module in the standard library for filesystem access and GitPython from PyPI for git access.