The Justice League doesn't need an origin story, just start the movie with them already together. Even if you've never read a comic, you're already going to know who these people are. Superman, Batman, Wonder Woman; the Flash to a certain extent, maybe Green Lantern. As Christopher said, the rest can be easily figured out as you go along. Yeah, they're the Super Friends. So what? They've gotten a shout-out on The Big Bang Theory almost every week for years now. This is DC's great advantage over Marvel, they don't need to set up the characters with solo movies in advance. Just go for it. They're the goddamn Justice League.