Added docs about custom alert sounds on iOS

**Note:** Local sound files must be placed into the res-folder and not into the assets-folder.

### Notification sound on iOS

The sound must be located in your project's resources and must be a caf file.

You can package the audio data in an *aiff*, *wav*, or *caf* file. Then, in Xcode, add the sound file to your project as a nonlocalized resource of the application bundle. You may use the *afconvert* tool to convert sounds.

```javascript

/**

 * Plays the sound if the notification pop's up

 */

window.plugin.notification.local.add({ sound: 'sub.caf' });

```

**Note:** The right to play notification sounds in the notification center settings has to be granted.

**Note:** The right to play notification sounds in the notification center settings has to be granted.<br>

**Note:** Custom sounds must be under 30 seconds when played. If a custom sound is over that limit, the default system sound is played instead.

### LiveTile background images on WP8

LiveTile's have the ability to display images for different sizes. These images can be defined through the `smallImage`, `image` and `wideImage` properties.<br>