The feature was first announced in early November when Mojang revealed its partnership with Twitch to bring built-in broadcasting to the game. The feature is now live and is supported on Windows (XP or higher) and Mac OS X Lion (10.7).
Mojang wrote on its website that the functionality may still be a bit buggy and not all features have been implemented yet, but it should still give users the opportunity to test it and provide feedback.
Instructions on how to use the broadcasting integration can be viewed here. The video above shows a walkthrough of the process.