add documentation on how to do data regeneration and use this repository

[tbuckl]

this will help new users understand how to generate necessary data and to use this repository

[tbuckl]

This was a sloppy pull request of things that were complex and in progress and finished, so we broke it out into a pull request that could be closed:

https://github.com/synthicity/bayarea_urbansim/pull/51

an in-progress branch:

https://github.com/MetropolitanTransportationCommission/bayarea_urbansim/tree/data/refactor-regeneration

and several updates to MTC's setup documentation:

https://github.com/MetropolitanTransportationCommission/bayarea_urbansim_setup/commit/179c9b8ab142046d9af54176bbd4b1f686ec4cd4

https://github.com/MetropolitanTransportationCommission/bayarea_urbansim_setup/commit/454db31c04d56aa1a9df31fa4efbe78a4c89dfaf