The upcoming OS X 10.11 El Capitan release should contain a new feature called photo extensions. There’s even some UI in place already, for example in the Photos app:
But the documentation for this feature is nowhere to be found. Where should I look?
Documentation hasn't been updated but it works exactly the same as on iOS. It's just about adding a protocol support + NSExtension dictionary into your Info.plist Add TARGET (not create new project) to your project like here and use iOS documentation. The XCode7 template has everything you need. No UIKit stuff there
