update readme
update readme

R=eernst@google.com

Review URL: https://codereview.chromium.org/988083002

google/reflectable.dart,jakemac53/reflectable,luisvt/reflectable,google/reflectable.dart,bwu-dart-contributing/reflectable,jakemac53/reflectable,luisvt/reflectable,bwu-dart-contributing/reflectable