Update contributing docs (#416). (#418)
Update contributing docs (#416). (#418)

* Update contributing docs.

* improve git flow instructions
* add review protocol
* add note about "WIP" conventions

* Update CONTRIBUTING.md

* Update CONTRIBUTING.md

alexeieleusis/dart-lang_linter,alexeieleusis/dart-lang_linter,dart-lang/linter,dart-lang/linter