Document preference value "KeepAVAudioSessionAlwaysActive" used in config.xml

[jxxmin]

Feature Request

Motivation Behind Feature

Saves many hours of debugging!

Feature Description

There must be a preference value set in the config.xml file in order to be able to switch to another audio track when playing music in the background on iOS. This should be written in the documentation of the plugin.

<platform name="ios"><preference name="KeepAVAudioSessionAlwaysActive" value="YES" /> ...