The old saw is true that “there’s no such thing as a free lunch.” But at least for today, the money you spend can go to a good cause. As part of their opening festivities, Roti Mediterranean Grill (1747 Pennsylvania Ave. NW) is giving away free lunches, but requesting donations be made to D.C. Central Kitchen. The non-profit trains formerly homeless men and women how to cook, using donated surplus food to feed other…