By Veronika Bondarenko The Mexican chain just launched its new “Guac Mode” benefit. Gone are the days of simply putting out a tasty product and hoping people will come — the new fast food frontier is bringing as many customers online as possible. That was what pushed Wendy’s (WEN) – Get Wendy’s Company Report to entice app downloads with free fries and nuggets and Yum! Brands (YUM) – Get Yum! Brands, Inc. Report-owned Taco Bell to launch its app-only taco-a-day-for-$10 subscription. While McDonald’s (MCD) – Get McDonald’s Corporation Report’s ubiquity helps its dominate the app world (2.2 mill…