Who should be credited as an author in pubspec.yaml?
It's common to have all people who contributed anything listed, even fixes to typos in README or code files, but it's completely up to the project owner.
It's probably a good idea to add a CONTRIBUTING.md file to your project and mention the rules you want to apply to your project. You could also mention that people who are interested include themselves to authors in pubspec.yaml so you don't need to maintain it yourself.