Add notice about canonical source NO CHANGELOG

README.md

@@ -7,6 +7,20 @@ push/pull and Git archive downloads.
 Workhorse itself is not a feature, but there are [several features in
 GitLab](doc/architecture/gitlab_features.md) that would not work efficiently without Workhorse.
 
+## Canonical source
+
+The canonical source for Workhorse is currently
+[gitlab-org/gitlab-workhorse](https://gitlab.com/gitlab-org/gitlab-workhorse).
+As explained in https://gitlab.com/groups/gitlab-org/-/epics/4826, we
+are in the process of moving the canonical source to
+[gitlab-org/gitlab/workhorse](https://gitlab.com/gitlab-org/gitlab/tree/master/workhorse).
+
+Until that transition is complete, changes (Merge Requests) for
+Workhorse should be submitted at
+[gitlab-org/gitlab-workhorse](https://gitlab.com/gitlab-org/gitlab-workhorse).
+Once merged, they will propagate to gitlab-org/gitlab/workhorse via
+the usual Workhorse release process.
+
 ## Documentation
 
 Workhorse documentation is available in the [`doc` folder of this repository](doc/).