qmlplugindump: Add explanation why it needs to be an app bundle

Since this is popping up quite regularly, add an explanation why
we need qmlplugindump to be an application bundle.

Change-Id: Iacf5ad0742aa9fb601c3be046f268764df062e81
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>