Give link to additional information in "Platform support" section

[sigurdm]

Sometimes our platform support detection fails with a false negative. In that case you are supposed to override the "platforms" section in your pubspec.yaml. But the scoring section is currently not very helpful:

See eg. https://pub.dev/packages/flutter_widget_from_html/score

We could perhaps just add links to https://dart.dev/tools/pub/pubspec#platforms https://docs.flutter.dev/packages-and-plugins/developing-packages#plugin-platforms