Cameron Crowe’s documentary “The Union” will have its world premiere as the opening night attraction at the 10th Tribeca Film Festival on April 20. A performance by the film’s subject, Elton John, will follow the free outdoor screening at the North Cove of the World Financial Plaza. “Almost Famous” director Crowe spent a year filming John’s collaboration with Leon Russell on an album produced by T-Bone Burnett. The Tribeca fest, which runs through May 1, will announce its slate next Monday and Tuesday. The full release is at IndieWire.