message: make the contributing checklist more explicit
diff: [CODE OF CONDUCT](https://github.com/lumeland/lume/blob/master/CODE_OF_CONDUCT.md)
  - [ ] Have you read the document
  [CONTRIBUTING](https://github.com/lumeland/lume/blob/master/CONTRIBUTING.md)
  +
  + - [ ] One pull request per feature. If you want to do more than one thing,
  + send multiple pull request.
  + - [ ] Write tests.
  + - [ ] Run deno `fmt` to fix
  + the code format before commit.
  + - [ ] Document any change in the
  + `CHANGELOG.md`.
  