Add/document support for Multiple APK and APK Extension files

[aginiewicz]

We should investigate how to make multiple apk files using our tool-chain from one project to distribute special versions or (more important) resource packs optimized for different devices and/or patch releases.

• http://developer.android.com/google/play/publishing/multiple-apks.html
• http://developer.android.com/google/play/expansion-files.html

[ajhager]

Is this something you are still interested in? Any progress with it?