I see that Adobe has now published a step-by-step guide, but right at the very beginning they say they're working on a migration tool (good) but that the step-by-step guide they're putting out now will (essentially) completely ruin your photo collection, since the list of things it can't import is LONG, starting with your edited images. You have to export all those as TIFF so now you have two copies of all your pictures. No thanks!
Aperture will continue to work in Yosemite, as will iPhoto, and both should be compatible with the forthcoming Photos. While it may not be the right solutionfor everyone, if Macphun and Athentech and perhaps a couple others offer plug-ins for Photos, I'm in.