ci(gitlab): add commit lint and its documentation

See merge request !2

* .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.

ci(codeowners): define @EOLE group members as owners

See merge request !3

This permit to notify the group for reviewing merge requests.