update doc for docker image (#353)

* update doc for docker image

* update

deployment/docker/README.md

@@ -30,4 +30,7 @@ __Run the docker image__
 or
 ```
     docker run --runtime=nvidia -it nni/nni
-```
\ No newline at end of file
+```
+
+## 3.Directly retrieve the docker image
+If you do not want to build docker image, you can retrieve the NNI docker image from Docker Hub through the command `docker pull msranni/nni:latest`.
\ No newline at end of file

docs/InstallNNI_Ubuntu.md

@@ -22,7 +22,7 @@
 
 * __Install NNI in docker image__
 
-    You can also install NNI in a docker image. Please follow the instructions [here](../deployment/docker/README.md) to build NNI docker image.
+    You can also install NNI in a docker image. Please follow the instructions [here](../deployment/docker/README.md) to build NNI docker image. The NNI docker image can also be retrieved from Docker Hub through the command `docker pull msranni/nni:latest`.
 
 ## Further reading
 * [Overview](Overview.md)