adding notes for upgrading to polymer 0.5.2
adding notes for upgrading to polymer 0.5.2

ronlobo/dart-tutorials-samples,ronlobo/dart-tutorials-samples,dart-archive/dart-tutorials-samples,dart-archive/dart-tutorials-samples,kasperpeulen/dart-tutorials-samples,dart-archive/dart-tutorials-samples,kasperpeulen/dart-tutorials-samples,ronlobo/dart-tutorials-samples,kasperpeulen/dart-tutorials-samples