How to use commnents in Dart code?

Add a section in README to describe how to use comments in Dart code

- line comment with `//`
- block comment `/*...*/`
- documentation comment with `///`. Can be useful with [dartdoc](https://github.com/dart-lang/dartdoc) which is a tool to convert doc comment to html. Markdown format can be used in doc comment

ref: https://dart.dev/guides/language/effective-dart/documentation