Improve fullscreen documenation (#364)

// on a single screen if you like.

<Video source={{uri: "background"}}   // Can be a URL or a local file.

       ref={(ref) => {

         this.player = ref

       }}                             // Store reference

       rate={1.0}                     // 0 is paused, 1 is normal.

       volume={1.0}                   // 0 is muted, 1 is normal.

       muted={false}                  // Mutes the audio entirely.

       onError={this.videoError}      // Callback when video cannot be loaded

       style={styles.backgroundVideo} />

// Later to trigger fullscreen

this.player.presentFullscreenPlayer()

// To set video position in seconds (seek)

this.player.seek(0)

// Later on in your styles..

var styles = StyleSheet.create({

  backgroundVideo: {

Seeks the video to the specified time (in seconds). Access using a ref to the component

`presentFullscreenPlayer()`

Toggles a fullscreen player. Access using a ref to the component.

## Examples

- See an [Example integration][1] in `react-native-login` *note that this example uses an older version of this library, before we used `export default` -- if you use `require` you will need to do `require('react-native-video').default` as per instructions above.*