Add link to ultraloop in INSTALL.md

Please consider mentioning https://github.com/T045T/ultraloop in https://github.com/marcan/blitzloop/blob/master/docs/INSTALL.md as a way to turn UltraStar files (which are way easier to find) into `.blitz`. Otherwise potential users may start looking elsewhere once they found out `.blitz` file format is not widely supported (personal experience).