Add the client design doc. (#2073)

* Add client_tool design doc.

* Do some rephrase

* Do some rephrase

* Update the bullet.

Author: brightcoder01

docs/designs/client_tool.md

@@ -0,0 +1,83 @@
+# ElasticDL Command-line Client Tool
+
+## Background
+
+ElasticDL is a Kubernetes-Native deep learning framework. As it runs
+distributed training/prediction/evaluation jobs in a cluster, we need a client
+to submit the jobs to the cluster. The main functionality of the client is
+*building image for ElasticDL job* and *submitting ElasticDL job*.
+
+Currently we have a client but it's tightly coupled with the main package. It's
+too heavy that users need pip install the whole elasticdl package and lots of
+dependencies such as TensorFlow, grpcio, etc.
+
+To improve the user experience, the client should be light-weight. It only has
+depedency on docker and Kubernetes Api. In this doc, we are discussing about
+this command-line client tool.
+
+## User Story
+
+1. Users develop model and the directory structure of model definition files
+   is as follows:
+
+    ```TEXT
+    a_directory
+    - wide_and_deep.py
+    requirements.txt
+    ```
+
+1. Generate a Dockerfile.  
+
+    Input the command:
+
+    ```bash
+    cd ${model_root_path}
+    elasticdl zoo init [base_image_name]
+    ```
+
+    `base_image_name` is optional and the default value is `python`.
+    The generated Dockerfile example is:
+
+    ```Dockerfile
+    FROM python
+    COPY . /model_zoo
+    RUN pip install -r /model_zoo/requirements.txt
+    RUN pip install elasticdl
+    ```
+
+    Users can make additional updates on the Dockerfile if necessary.
+
+1. Build the Docker image for an ElasticDL job.
+
+    ```bash
+    elasticdl zoo build --image=a_docker_registry/bright/elasticdl-wnd:1.0 .
+    ```
+
+1. Push the Docker image to a remote registry (optional)
+
+    ```bash
+    elasticdl zoo push a_docker_registry/bright/elasticdl-wnd:1.0
+    ```
+
+1. Submit a model training/prediction/evaluation job.
+
+    ```bash
+    elasticdl train \
+        --image=a_docker_registry/bright/elasticdl-wnd:1.0 \
+        --model_def=a_directory.wide_and_deep.custom_model \
+        --training_data=/data/mnist/train \
+        --validation_data=/data/mnist/test \
+        --num_epochs=2 \
+        --minibatch_size=64 \
+        --num_ps_pods=1 \
+        --num_workers=1 \
+        --evaluation_steps=50 \
+        --job_name=test-mnist \
+        --distribution_strategy=ParameterServerStrategy \
+        --master_resource_request="cpu=0.2,memory=1024Mi" \
+        --master_resource_limit="cpu=1,memory=2048Mi" \
+        --worker_resource_request="cpu=0.4,memory=1024Mi" \
+        --worker_resource_limit="cpu=1,memory=2048Mi" \
+        --ps_resource_request="cpu=0.2,memory=1024Mi" \
+        --ps_resource_limit="cpu=1,memory=2048Mi"
+    ```