docs(camera-preview): add usage
docs(camera-preview): add usage

closes #363

driftyco/ionic-native,ramonornela/ionic-native,xueron/ionic-native,driftyco/ionic-native,xueron/ionic-native,klirix/ionic-native,ramonornela/ionic-native,klirix/ionic-native,xueron/ionic-native,klirix/ionic-native,ramonornela/ionic-native,driftyco/ionic-native,xueron/ionic-native,ramonornela/ionic-native