Add dedicated info about artifacts:untracked to GitLab CI's README.md

What does this MR do?

Add dedicated info about the CI artifacts:untracked

Are there points in the code the reviewer needs to double check?

Just documentation

Why was this MR needed?

For me, it was hard to understand, what artifacts:untracked do – because I found infos about suboptions for artifacts, but no info about untracked.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?