docs(commitlint): describe usage of the commitlint job

Verify formatting of all commit messages in the range
`$BASE_BRANCH..$CI_COMMIT_SHA`.

USAGE
=====

include:
  - project: EOLE/infra/ci-tools
    ref: stable
    file: /templates/Rules.yaml
  - project: EOLE/infra/ci-tools
    ref: stable
    file: /templates/Lint/Commitlint.yaml

stages:
  - lint

OPTIONAL VARIABLES
==================

- `BASE_BRANCH`: name of the referrence branch, defaults to
  `${CI_DEFAULT_BRANCH}`
- `COMMITLINT_IMAGE`: name of the `commitlint` docker image to use

REQUIREMENTS
============

- The `.not-on-stable` rules template
- A `lint` stage must be present in your pipeline or it must be
  overriden by the extending job to feet your need.

USED CI VARIABLES
=================

- `CI_COMMIT_SHA`
- `CI_DEFAULT_BRANCH`