update doc (#2363)
update doc (#2363)

related to https://github.com/react-community/react-native-maps/issues/2237#issuecomment-402487329
lelandrichardson/react-native-maps,lelandrichardson/react-native-maps,wannyk/react-native-maps,wannyk/react-native-maps,wannyk/react-native-maps,wannyk/react-native-maps,lelandrichardson/react-native-maps