Add a note about GitLab CI/CD for external repositories to the README
Premium users can use the mirror feature in GitLab to stay up-to-date.
Note that while "Starter" instances have the Pull Mirroring feature available, it looks like you need Premium or Ultimate to unlock CI with it. https://about.gitlab.com/2018/03/22/gitlab-10-6-released/#gitlab-cicd-for-external-repos
For GitLab.com, it starts from a Silver subscription.