Add free pull mirroring expiry note

doc/workflow/repository_mirroring.md

@@ -101,6 +101,9 @@ The repository will push soon. To force a push, click the appropriate button.
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/51) in GitLab Enterprise Edition 8.2.
 > [Added Git LFS support](https://gitlab.com/gitlab-org/gitlab-ee/issues/10871) in [GitLab Starter](https://about.gitlab.com/pricing/) 11.11.
 
+NOTE: **Note:** This feature [is available for free](https://gitlab.com/gitlab-org/gitlab-ee/issues/10361) to
+GitLab.com users until September 22nd, 2019.
+
 You can set up a repository to automatically have its branches, tags, and commits updated from an
 upstream repository.