Add note about executor for managed cluster apps

doc/user/clusters/applications.md

@@ -667,6 +667,10 @@ To install applications using GitLab CI/CD:
      - template: Managed-Cluster-Applications.gitlab-ci.yml
    ```
 
+   Note that the job provided by this template connects to the cluster using tools provided 
+   in a custom Docker image. It requires that you have a runner registered with the Docker, 
+   Kubernetes, or Docker Machine executor.
+
 1. Add a `.gitlab/managed-apps/config.yaml` file to define which
   applications you would like to install. Define the `installed` key as
   `true` to install the application and `false` to uninstall the