Update README.md YAML ref to denote a limitation with anchors & include

What does this MR do?

Stumbled over this limitation during usage. Add the limitation note that anchors & include cannot be used in combination if the anchor and tag cross file boundary.

This is in reference to:

• https://docs.gitlab.com/ee/ci/yaml/#anchors &
• https://docs.gitlab.com/ee/ci/yaml/#include

Basic investigation

• https://yaml.org/spec/1.2/spec.html#id2762107
• https://stackoverflow.com/questions/55304806/gitlab-and-placing-hidden-key-scripts-into-include-files

Related issues

TBD (unknown) it is difficult to search/find if this is a known issue, or if we should or should not submit a report for it. (it would be NICE if it worked ... )

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.