Update URL for LOC Thomas in Readme

[spswanz]

The Library of Congress's THOMAS website (http://thomas.loc.gov/) has been retired since 2012 and the redirects no longer work. The data is now at Congress.gov

https://www.congress.gov/help/faq#thomasretirement

[JoshData]

I think you're referring to the documentation for the thomas_id field. Those numeric ids aren't on Congress.gov, so it wouldn't be right to update the URL We should just mention that it's obsolete. It remains in our data for historical purposes.