[Readme] Explain how to import for Swift
[Readme] Explain how to import for Swift

hanangellove/MRProgress,yungfan/MRProgress,boherna/MRProgress,AlexanderMazaletskiy/MRProgress,jackywpy/MRProgress,zaubara/MRProgress,odyth/MRProgress,yungfan/MRProgress,odyth/MRProgress,boherna/MRProgress,jackywpy/MRProgress,colemancda/MRProgress,yarry/MRProgress,colemancda/MRProgress,AlexanderMazaletskiy/MRProgress,AddAloner/MRProgress,hanangellove/MRProgress,AddAloner/MRProgress,mrackwitz/MRProgress,mrackwitz/MRProgress,zaubara/MRProgress,yarry/MRProgress,zaubara/MRProgress