Added docs for similar issues

doc/user/project/issues/index.md

@@ -155,3 +155,7 @@ Read through the [API documentation](../../../api/issues.md).
 ### Bulk editing issues
 
 Find out about [bulk editing issues](../../project/bulk_editing.md).
+
+### Similar issues
+
+Find out about [similar issues](similar_issues.md).

doc/user/project/issues/similar_issues.md

+# Similar issues
+
+> [Introduced][ce-22866] in GitLab 11.6.
+
+Similar issues suggests issues that are similar when new issues are being created.
+This features requires [GraphQL] to be enabled.
+
+![Similar issues](img/similar_issues.png)
+
+You can see the similar issues when typing in the title in the new issue form.
+This searches both titles and descriptions across all issues the user has access
+to in the current project. It then displays the first 5 issues sorted by most
+recently updated.
+
+[GraphQL]: ../../../api/graphql/index.md
+[ce-22866]: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22866