Add Carthage badge to README.
Add Carthage badge to README.

gonzalonunez/Parse-SDK-iOS-OSX,ParsePlatform/Parse-SDK-iOS-OSX,gonzalonunez/Parse-SDK-iOS-OSX,ParsePlatform/Parse-SDK-iOS-OSX,podkovyrin/Parse-SDK-iOS-OSX,podkovyrin/Parse-SDK-iOS-OSX