Update
It's official, if you preorder the game you get the original Dark Souls for free, via backwards compatibility. All the details are available over on the Xbox Store.
Original
Bandai Namco and Microsoft seems to have planned a pretty nice pro-order bonus for all Xbox One gamers who get Dark Souls III. Everyone purchasing the game will get a copy of the original Dark Souls for free, which of course can be played on Xbox One thanks to backwards compatibility.
This was revealed via the Australian Xbox.com, but when media noticed this it was hastily removed. Fortunately (?), it's hard to remove things from the internet and pictorial proof can be found below. Remember that this is Aussie dollars, not American, hence the seemingly high price.
It's also worth noting that Dark Souls isn't playable on Xbox One right now, but this is more or less confirmation that it will arrive fairly shortly since Dark Souls III will be released on April 12.
Thanks, ExlGaming