Document that Mac bindings can only be built on a Mac

[BioTurboNick]

Would be great to document that Mac bindings can only be built on a Mac, and that Pair to Mac will not work.

Better if it could be documented which built files from the Mac need to be moved back over to Windows, and how to incorporate them into the solution.

[BioTurboNick]

Okay the Firebase example doesn't build on a Mac either because it can't resolve the Firebase package dependencies. I'm not familiar with XCode but the docs don't seem to have steps I missed.