Clarify that Auto Deploy requires a public project.

doc/ci/autodeploy/index.md

 # Auto deploy
 
-> [Introduced][mr-8135] in GitLab 8.15.
+> [Introduced][mr-8135] in GitLab 8.15. Currently requires a [Public project][project-settings].
 
 Auto deploy is an easy way to configure GitLab CI for the deployment of your
 application. GitLab Community maintains a list of `.gitlab-ci.yml`
@@ -33,6 +33,7 @@ enable [Kubernetes service][kubernetes-service].
 created automatically for you.
 
 [mr-8135]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8135
+[project-settings]: https://docs.gitlab.com/ce/api/projects.html#project-visibility-level
 [project-services]: ../../project_services/project_services.md
 [auto-deploy-templates]: https://gitlab.com/gitlab-org/gitlab-ci-yml/tree/master/autodeploy
 [kubernetes-service]: ../../project_services/kubernetes.md