Dear Mr. Smith ?
I have an idea for a documentary which I believe is perfect for the XX network/festival circuit/whatever-marketplace, about (insert your idea here). I have developed a very brief proposal which, with your permission, I would like to send you. I am not interested in making money from this idea, and would gladly release any rights which I may hold should you decide to take up the project. Thanks for your attention. Sincerely, etc.