Update the tutorial of the end-to-end demo

mnist-client/README.md

 # MNIST classification by PaddlePaddle
 
-Forked from https://github.com/sugyan/tensorflow-mnist
-
 ![screencast](https://cloud.githubusercontent.com/assets/80381/11339453/f04f885e-923c-11e5-8845-33c16978c54d.gif)
 
-## Build
+## Usage
 
-    $ docker build -t paddle-mnist .
+This MNIST classification demo consists of two parts: a PaddlePaddle
+inference server and a Javascript front end. We will start them
+separately.
 
-## Usage
+We will use Docker to run the demo, if you are not familiar with
+Docker, please checkout
+this
+[tutorial](https://github.com/PaddlePaddle/Paddle/wiki/TLDR-for-new-docker-user).
+
+### Start the Inference Server
+
+The inference server can be used to inference any model trained by
+PaddlePaddle. Please see [here](TODO) for more details.
+
+1. Download the MNIST inference model topylogy and parameters to the
+   current working directory.
+
+    ```bash
+    wget https://s3.us-east-2.amazonaws.com/models.paddlepaddle/inference_topology.pkl
+    wget https://s3.us-east-2.amazonaws.com/models.paddlepaddle/param.tar
+    ```
+
+1. Run following command to start the inference server:
+
+    ```bash
+    docker run --name paddle_serve -v `pwd`:/data -d -p 8000:80 -e WITH_GPU=0 paddlepaddle/book:serve
+    ```
+
+    The above command will mount the current working directory to the
+    `/data` directory inside the docker container. The inference
+    server will load the model topology and parameters that we just
+    downloaded from there.
 
+    After you are done with the demo, you can run `docker stop
+    paddle_serve` to stop this docker container.
+
+### Start the Front End
+
+1. Run the following command
+    ```bash
+    docker run -it -p 5000:5000 paddlepaddle/book:mnist
+    ```
+
+1. Visit http://localhost:5000 and you will see the PaddlePaddle MNIST demo.
+
+
+## Build
+
+We have already prepared the pre-built docker image
+`paddlepaddle/book:mnist`, here is the command if you want build the
+docker image again.
 
-1. Download `inference_topology.pkl` and `param.tar` to current directory
-1. Run following commands:
 ```bash
-docker run -v `pwd`:/data -d -p 8000:80 -e WITH_GPU=0 paddlepaddle/book:serve
-docker run -it -p 5000:5000 paddlepaddle/book:mnist
+docker build -t paddlepaddle/book:mnist .
 ```
-1. Visit http://localhost:5000
+
+
+## Acknowledgement
+
+Thanks to the great project https://github.com/sugyan/tensorflow-mnist
+. Most of the code in this project comes from there.