ci: improve `report-job-status` README

This patch adds a warning about using the action in pull requests
created from forks. GitHub secrets are not passed to workflows in
this case [1] and due to this limitation the action will not work
correctly for such pull requests.

[1] https://docs.github.com/en/actions/security-guides/encrypted-secrets#using-encrypted-secrets-in-a-workflow

NO_DOC=ci
NO_TEST=ci
NO_CHANGELOG=ci