Lightning Connector Power Output rating/ Apple Policy
If you work for a company that is willing to pay something you might have a look at the MFi program. I don't have a clue about pricing but you might try to ask them.

If you work on your own and don't have a company, or you don't want to pay for the spec you either start designing something else or try to guess the spec reading around... That is probably a bad idea. You might also try to search the spec on the net but you'd probably end up with an unauthorized copy and I believe that apple would not be willing to certify your device in such a case. If you are designing a sort of hack for home use this might be a feasible way but remember that the spec is probably copyrighted so downloading it is probably illegal.