Add clearer documentation about runtime arguments

[yoichitgy]

Add documentation as mentioned by @brunopinheiro in Issue #45.

class PetOwner {
   var pet: AnimalType
}

container.register(PetOwner.self) { (_: ResolverType, pet: AnimalType) in
   var petOwner = PetOwner()
   petOwner.pet = pet
   return petOwner
}

var cat = Cat(name: "Bilbo")

// Won't work, since we don't have any Registration Key matching [PetOwner, (Cat) -> PetOwner]
var petOwner = container.resolve(PetOwner.self, argument: cat)!

// This is the correct Registration Key [PetOwner, (AnimalType) -> PertOwner]
var petOwner = container.resolve(PetOwner.self, argument: cat as AnimalType)!

[yoichitgy]

Add the documentation by 5b2d047.