Merge branch 'docs-cicd-note' into 'master'

Add promotion note for GL.com users

Closes #9545

See merge request gitlab-org/gitlab-ee!9430

doc/ci/ci_cd_for_external_repos/index.md

 # GitLab CI/CD for external repositories **[PREMIUM]**
 
+NOTE: **Note:**
+To [promote its release](https://about.gitlab.com/2018/03/22/gitlab-10-6-released/#gitlab-cicd-for-external-repos), GitLab.com users are receiving this feature for free through March 22, 2019.
+
 >[Introduced][ee-4642] in [GitLab Premium][eep] 10.6.
 
 GitLab CI/CD can be used with GitHub or any other Git server.