Document USDA ERS data

[mizmay]

Rural-Urban Continuum codes and 2020 census data are incorporated into the data source but not currently being used for anything: https://www.ers.usda.gov/data-products/rural-urban-continuum-codes/

[mizmay]

I'd rather be using this classification, as it does a better job of distinguishing suburbs but it's not updated for 2020 (yet - says coming in 2024!)

https://www.cdc.gov/nchs/data-analysis-tools/urban-rural.html