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>

tools/qmlplugindump/qmlplugindump.pro

 QT += qml qml-private quick-private core-private
 
+#
+# qmlplugindump is an applicaton bundle on the mac
+# so that we can include an Info.plist, which is needed
+# to surpress qmlplugindump popping up in the dock
+# when launched.
+#
+
 CONFIG += qpa_minimal_plugin
 
 SOURCES += \