Add GitLab CI usage example to README.md

[orhun]

Is your feature request related to a problem? Please describe.
It would be nice to see an example in the documentation for the usage of git-cliff with GitLab CI.

Describe the solution you'd like
Add an example .gitlab-ci.yml to README.md, preferably after the github-action section.

Describe alternatives you've considered
None

Additional context
#20

[alteregoart]

changelog:
  image:
    name: orhunp/git-cliff:latest
    entrypoint: [""]
  stage: doc
  script:
    - git-cliff -r . > CHANGELOG.md
  artifacts:
    paths:
      - CHANGELOG.md 

I think this is the minimum configuration
@orhun

[alteregoart]

and this solution work fine on this repo https://gitlab.com/alteregoart/git-cliff-test (on the docs.gitlab-ci.yml)

[orhun]

and this solution work fine on this repo gitlab.com/alteregoart/git-cliff-test (on the docs.gitlab-ci.yml)

Awesome! Do you mind submitting a PR to add this example to README.md?

[alteregoart]

Nope
I did, see #24