Add link to issue for MySQL/subgroups drop support

doc/user/group/subgroups/index.md

@@ -2,7 +2,8 @@
 
 >**Notes:**
 - [Introduced][ce-2772] in GitLab 9.0.
-- MySQL support for subgroups was dropped in GitLab 9.3.
+- MySQL support for subgroups was dropped in GitLab 9.3
+  [due to performance reasons][issue].
 
 With subgroups (aka nested groups or hierarchical groups) you can have
 up to 20 levels of nested groups, which among other things can help you to:
@@ -175,3 +176,4 @@ Here's a list of what you can't do with subgroups:
 [ce-2772]: https://gitlab.com/gitlab-org/gitlab-ce/issues/2772
 [permissions]: ../../permissions.md#group
 [reserved]:  https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/path_regex.rb
+[issue]: https://gitlab.com/gitlab-org/gitlab-ce/issues/30472#note_27747600