update base image

README.md

@@ -11,11 +11,12 @@ One of the key features of this project is efficient memory coordination, enabli
 
 ### Build from Dockerfile
 
-Clone this repository:
+1. Get or build base image (base with customized xpytorch, ops, etc.). Ref: [installation](https://vllm-kunlun.readthedocs.io/en/latest/installation.html).
 
-```bash
-docker build -t $build_image -f ./Dockerfile .
-```
+2. Clone this repository and build
+    ```bash
+    docker build -t $build_image -f ./Dockerfile .
+    ```
 
 ## Usage