Update Readme.md — (not a) MAC address on iOS
Update Readme.md — (not a) MAC address on iOS

As MAC address is not obtainable on iOS the `deviceAddress` should be addressed as `device identifier` that is different on iOS.
Closes Pilloxa/react-native-nordic-dfu#18
Pilloxa/react-native-nordic-dfu,Pilloxa/react-native-nordic-dfu,Pilloxa/react-native-nordic-dfu