link new doc from indexes pages

doc/README.md

@@ -85,6 +85,7 @@ Manage files and branches from the UI (user interface):
 - [Discussions](user/discussions/index.md) Threads, comments, and resolvable discussions in issues, commits, and  merge requests.
 - [Issues](user/project/issues/index.md)
 - [Issue Board](user/project/issue_board.md)
+- **(EES/EEP)** [Related Issues](user/project/issues/related_issues.md): create a relationship between issues
 - [Issues and merge requests templates](user/project/description_templates.md): Create templates for submitting new issues and merge requests.
 - [Labels](user/project/labels.md): Categorize your issues or merge requests based on descriptive titles.
 - [Merge Requests](user/project/merge_requests/index.md)

doc/user/project/issues/index.md

@@ -132,6 +132,14 @@ Issues can be [exported as CSV](csv_export.md) from GitLab and are sent to your
 
 _Exporting issues to CSV is available only in [GitLab Enterprise Edition](https://about.gitlab.com/gitlab-ee/)._
 
+## Related Issues (EES/SSP)
+
+Related Issues are a bi-directional relationship between any two issues
+and appear in a block below the issue description. Issues can be across groups
+and projects.
+
+Read more about [Related Issues](related_issues.md).
+
 ### Issue's API
 
 Read through the [API documentation](../../../api/issues.md).