A more fitting debut could not have been conceived for the new feature documentary “How to Start a Revolution,” given its world premiere on September 18th as part of the 27th annual Boston Film Festival. In attendance were the director, Ruaridh Arrow, as well as a few of the people featured in the film: Robert Helvey, Jamila Raqib, and the man himself, Gene Sharp. At 83 years old and with rather limited mobility, Dr. Sharp rarely makes public appearances these days. But the several hundred who had turned out to see him in Boston were by no means disappointed, responding with at least three standing ovations on the afternoon. For those of us lucky enough to have been there and hear him speak, including a number of his close friends and colleagues, it was impossible not to recognize the deep significance of the moment, with the humble Dr. Sharp visibly moved by the outpouring of support.