Update README with clarified custom font usage (#818)
Update README with clarified custom font usage (#818)

* clarifying custom icomoon font usage

- clarify `fontFile` argument
- provide example of custom font usage with android support

* update explicit IcoMoon example

* Adding IcoMoon Link

oblador/react-native-vector-icons,oblador/react-native-vector-icons,oblador/react-native-vector-icons,oblador/react-native-vector-icons,oblador/react-native-vector-icons