Iterate on readme

Just some proposed clarifications to the documentation for the agent branch. Please see [the full README in the branch view](https://github.com/soundcloud/glimpse/blob/iterate-on-readme/README.md) for changes to diagrams. @xla @grobie — let me know what you think.
I find it easier to parse to put footnotes at end of section or paragraph, rather than end of document. Especially for lengthy documents like this one. But I don't have strong feelings, so if you still insist I'll change it :) Let me know.