LP#1384932: document the zips.txt ZIP code database feature

Patron Address City/State/County Pre-Populate by ZIP Code.

Documented the zips.txt feature that auto fills patron address
(City, State, County) information.

Signed-off-by: Josh Stompro <stomproj@larl.org>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>