Doc: Added a note about editing the default AndroidManifest.xml

Changes to the app premissions and features list also requires
the related comments to be removed from the manifest XML.
The deployment tool creates an apk with default permissions
and features if the comments are not removed.

Task-number: QTBUG-36927

Change-Id: I6af50f997db5939d68e8498b931f27da6bf02a5a
Reviewed-by: Topi Reiniö <topi.reinio@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>