Added beta alert to Geo documentation

doc/administration/geo/replication/object_storage.md

@@ -16,6 +16,9 @@ To have:
 
 > - [Introduced](https://gitlab.com/gitlab-org/gitlab/issues/10586) in GitLab 12.4.
 
+CAUTION: **Caution:**
+This is a [**beta** feature](https://about.gitlab.com/handbook/product/#beta) and is not ready yet for production use at any scale.
+
 **Secondary** nodes can replicate files stored on the **primary** node regardless of
 whether they are stored on the local filesystem or in object storage.