Merge branch 'docs/gitlab-com-artifacts-expire' into 'master'

Add artifacts expiry time for GitLab.com

See merge request gitlab-org/gitlab-ce!19520

doc/user/gitlab_com/index.md

@@ -60,6 +60,7 @@ Below are the current settings regarding [GitLab CI/CD](../../ci/README.md).
 | Setting                 | GitLab.com        | Default       |
 | -----------             | ----------------- | ------------- |
 | Artifacts maximum size  | 1G                | 100M          |
+| Artifacts [expiry time](../../ci/yaml/README.md#artifacts-expire_in)   | kept forever           | deleted after 30 days unless otherwise specified    |
 
 ## Repository size limit