Add Carthage support

[Jachobsen]

Thank you for providing this great resource!

I've been using my fork with Carthage for a few months now but don't see any reason why it shouldn't be a part of your repo, so everyone can choose to use Carthage.

It references the current swift file, so there shouldn't be any maintenance overhead.

See more at Supporting Carthage for your framework.

[mbogh]

Thank you for the effort, I will just try out the carthage support.

README.md should also be updated with Carthage instructions :) I can do this if you prefer.

[Jachobsen]

Done :)