Add CONTRIBUTING.md file
Add CONTRIBUTING.md file

google/protobuf.dart,dart-lang/protobuf,dart-lang/dart-protobuf,dart-lang/protobuf,dart-lang/protobuf,dart-lang/dart-protobuf,google/protobuf.dart,collinjackson/dart-protobuf,google/protobuf.dart,dart-lang/dart-protobuf