Add documentation for data repository purpose and layout

Last week during scrum, I described trying to clean up the layout of supporting code for the data repositories.  I implemented this for Iowa in https://github.com/openelections/openelections-data-ia/commit/fd44b35c1d181661a9eaadd2c4d6ee45143e260d and documented my thinking in this commit.

Let me know if this makes sense.

Also, let me know how this should be linked into the existing documentation.  I'm not sure if this supplants or extends http://docs.openelections.net/guide/#data-extraction