Operating system updates don't come quickly in the Android world. They show up fashionably late, nevermind that parties where everyone looks forward to a new software release aren't the kind where people care how anyone is dressed.
ASUS is now doing its users a solid. No, it's not pushing out over-the-air updates to all of its devices right this moment. But it is doing the next best thing, which is listing which phones will get the update.
ASUS hasn't provided an exact date, but it does say the update should arrive in the second quarter of this year. So hopefully you will have Marshmallow before the spring is up, just in time to get hyped for whichever dessert starts with N following Google I/O.