Add the Prerequisite part for elasticdl_client user story. (#2097)

* Add the Prerequisite part for elasticdl client user story.

* Update client_tool.md

* Update client_tool.md

Co-authored-by: Yi Wang <yi.wang.2005@gmail.com>

Author: brightcoder01

docs/designs/client_tool.md

@@ -17,21 +17,29 @@ this command-line client tool.
 
 ## User Story
 
+1. Prerequisite
+
+    - Install [Docker CE >= 18.x](https://docs.docker.com/docker-for-mac/install/)
+    for building the Docker images of the distributed ElasticDL jobs.
+    - Install Python >= 3.6.
+    - Install ElasticDL command-line tool by `pip install elasticdl_client`.
+
 1. Users develop model and the directory structure of model definition files
    is as follows:
 
     ```TEXT
-    a_directory
-    - wide_and_deep.py
-    requirements.txt
+    model_zoo_root
+        a_directory
+        - wide_and_deep.py
+        requirements.txt
     ```
 
 1. Generate a Dockerfile.  
 
     Input the command:
 
     ```bash
-    cd ${model_root_path}
+    cd model_zoo_root
     elasticdl zoo init
         [--base_image=customized_base_image_name]
         [--cluster_spec=prem_cluster_spec]