Valve have debuted Steam Broadcasting, a feature that lets you stream your game sessions and allow your friends and other Steam users to watch.
In order to try the broadcasting feature, you’ll need to opt into the Steam Client Beta. To do this, in the Steam client main menu, select Settings > Account and change your “Beta Participation” to “Steam Beta Update”.
Once you have access to the broadcasting feature, you can decide who gets to view your broadcasts. You can make it so your broadcasts are viewable by friends you invite, any friends on your list, friends that request to watch your game, or anyone that uses Steam. If you choose that last option, your broadcast will be available on the game’s hub.
A full FAQ of how the broadcasting feature works can be found here.