Document how to enable Crosplane with feature flag

doc/user/clusters/applications.md

@@ -417,6 +417,18 @@ install Crossplane using the
 [`values.yaml`](https://github.com/crossplaneio/crossplane/blob/master/cluster/charts/crossplane/values.yaml.tmpl)
 file.
 
+#### Enabling installation
+
+This is a preliminary release of Crossplane as a GitLab-managed application. By default,
+the ability to install it is disabled.
+
+To allow installation of Crossplane as a GitLab-managed application, ask a GitLab
+administrator to run following command within a Rails console:
+
+```ruby
+Feature.enable(:enable_cluster_application_crossplane)
+```
+
 ## Upgrading applications
 
 > [Introduced](https://gitlab.com/gitlab-org/gitlab-foss/merge_requests/24789) in GitLab 11.8.