This documentation provides:

- a reminder of some CI concepts
- setup of the project with access token for repository write
  permission
- setup of the `.gitlab-ci.yml` with a full featured example and a
  step by step setup

* README.md: explain the features and add a mermaid graph to show what
  is possible.

* .gitlab-ci.yml: define only stage `lint` for now and run
`commitlint` at that stage.

* docs/CONTRIBUTING.md: describe the commit message formatting rules.

* README.md: link to the contributing guide.