I may be mistaken, but I don’t think you’ll really need to know Forge. I’m hoping at any rate that plugins will be developed entirely on the Sponge platform, with no direct API use via Forge necessary, or even depending on Forge API in development workspaces. There isn’t much to Sponge at the moment, but @Tux2 has written a post about how to start a new Sponge plugin here. As far as I know, you can just depend on the Sponge API for everyone but the log4j in that tutorial.
Sorry, Tux if I’m pinging you a lot with these references to your setup guide XD