Merge branch 'docs/merge-request-pipelines' into 'master'

Add note about MR Pipeline Runner version

Closes gitlab-ee#11122

See merge request gitlab-org/gitlab-ce!27330

doc/ci/merge_request_pipelines/index.md

 # Pipelines for merge requests
 
+NOTE: **Note**:
+As of GitLab 11.10, pipelines for merge requests require GitLab Runner 11.9
+or higher due to the [recent refspecs
+changes](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25504).
+Anything lower will cause the pipeline to fail.
+
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/15310) in GitLab 11.6.
 
 Usually, when you create a new merge request, a pipeline runs with the