Adds a README and canonical example for treble_manifest_split. am: 70e8444410 am: 18065746ff am: efa11f1338 am: 1b18efe533

Original change: https://android-review.googlesource.com/c/platform/tools/treble/+/1689267

Change-Id: If20d028b328d879b9444a490e76378a0a97c72a5