message: append docker source info
diff: @@ -24,6 +24,19 @@ If `docker` is not found, please see [here](https://docs.docker.com/install/) fo
  After that, using `docker pull vesoft/nebula-graph:latest` to get `nebula` image and `docker images` that can display images status.
  +If `docker` is slow when pulling the image, configure an accelerator.
  +
  +You can add the source at `/etc/docker/daemon.json`, for Chinese users:
  +
  +```
  +{
  + "registry-mirrors": [
  + "https://dockerhub.azk8s.cn",
  + "https://reg-mirror.qiniu.com"
  + ]
  +}
  +```
  +
  ---
  ### Step 2 Startup Nebula Graph
  