County -> CSA/CBSA needs year, right?

[franknarf1]

The BLS moves counties between metro areas in some years, but it seems that the data set here is just a snapshot of the county assignments at some unspecified year. If that's correct, I think you should document which year it is or extend the data by adding a year column. In the latter case, I think the package would be very useful to a lot of folks working with Census data. Thanks!