The Tribeca Film Festival with start with a bang as Elton John gives a free concert performance on April 20 following a screening of “The Union,” a documentary about his musical collaboration with Leon Russell. The screening and concert will take place outdoors at the World Financial Center Plaza in Lower Manhattan, the festival announced today. The starting times have yet to be determined. The film festival runs from April 20 through May 1 in New York. “The Union,” directed by Cameron Crowe, chronicles how John and Russell made their highly praised 2010 album of the same name. The rest of the festival lineup will be announced on March 7 and March 14.