add image registry doc

Signed-off-by: myan <[email protected]>

README.md

@@ -326,8 +326,9 @@ Before deploying maestro to the Openshift cluster, we will first build image. By
 $ export container_tool=docker
 ```
 
-Then we will push the image to your OpenShift cluster default registry and then deploy it to the cluster. You need to follow [this document](https://docs.openshift.com/container-platform/4.13/registry/securing-exposing-registry.html) to expose a default registry manually and login into the registry with podman. You can also specific the image information with the environment variables `external_image_registry`, `image_repository` and `image_tag`
+Then we will push the image to your OpenShift cluster default registry and then deploy it to the cluster. You need to follow [this document](https://docs.openshift.com/container-platform/4.13/registry/securing-exposing-registry.html) to expose a default registry manually and login into the registry with podman. You can also customize the image with the environment variables `external_image_registry`, `image_repository` and `image_tag`. 
 
+> Note: The meastro image pulled during deploying comes from the `internal_image_registry`, be sure to specify the correct registry location.
 
 ```shell
 $ make deploy