point folks at a different library

[ethanpailes]

This library appears like it has not seen maintenance in a while, so It would be nice to add a note to the README pointing people to another library for doing this. The https://github.com/evansiroky/timezone-boundary-builder repo, which maintains up-to-date shapefiles seems to suggest a few options. https://github.com/ugjka/go-tz looks pretty reasonable to me.