Merge branch 'docs/extend-offer-external-ci-cd' into 'master'

Offer now until Sept 22, 2019

Closes #9706

See merge request gitlab-org/gitlab-ee!10367

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.
+This feature [is available for free](https://about.gitlab.com/2019/03/21/six-more-months-ci-cd-github/) to
+GitLab.com users until September 22nd, 2019.
 
 >[Introduced][ee-4642] in [GitLab Premium][eep] 10.6.