Add note about initialize_urls only populating NULL URLs

I just spent a short while wondering why calling `.initialize_urls` on my model was silently doing nothing. It was because when I added the column, I made it NOT NULL, so the URLs were initialized to empty strings, which are not returned by [`klass_previous_instances`](https://github.com/rsl/stringex/blob/ef196f92d88267b5402c4b37154cab733a763eec/lib/stringex/acts_as_url/adapter/active_record.rb#L14), so are not auto-populated. This pull request doesn't change that behavior, but adds a note to the README to hopefully help others avoid the same gotcha.