add a Contributing guide
add a Contributing guide

20matan/react-native-material-kit,xinthink/react-native-material-kit,xinthink/react-native-material-kit,xinthink/react-native-material-kit,rtmruczek/react-native-material-kit,rtmruczek/react-native-material-kit,xinthink/react-native-material-kit,20matan/react-native-material-kit,20matan/react-native-material-kit,rtmruczek/react-native-material-kit