Need description of columns and values in columns

[cthoyt]

trafficstars

Right now it's pretty hard to figure out what's actually in the database - it would be great to have an explanation of what each column is in the README as well as the kinds of data that might be in each (especially the categorical fields, like for the measurement types)

[davidlmobley]

What's here is literally what I received after his death, so if you're able to figure these out and do a PR with this type of info, I'd love to get it. :)