There’s no news on when the feature will properly launch, but if it’s currently in the beta stage and you can expect to see it pretty soon.

To use this function you will need to have your PC turned on and your steam client open and all you need to do is access the Steam store front through any browser at SteamPowered.com, and if you make a purchase, or even want to manage your downloads ahead of a scheduled night of gaming with a friend, then you can simply access your games list and install.

Once at your games list (found in community tab) simply tap install and it will begin miles away on your PC.
Whether this added function will be brought to the fancy Steam app or not it’s hard to say, but Valve would be missing a good trick if they didn’t.