There are of course many sources of information about interoperability. However there are none specifically about adding a Swift WatchKit target to an Objective-C iOS project.
Of course normally in the iOS project we go to File -> New Target -> WatchKit app -> select Swift or Obj-C and you're done.
Usually adding Swift files to an Obj-C project just requires a bridging-header file and Xcode can create it for you but in my case I am not adding a file.
Lastly, I will be using WatchConnectivity of course with local and remote notifications plus sending data back and forth communicating between both apps. What else should I be aware of?