Graph viz set up (#282)

* Include the Graph requirement, GraphViz` in docs.

* Provide information to the ONNX format model export.

* Ask the user to download Graphviz via the website instead of brew.

README.cn.md

@@ -130,6 +130,8 @@ board 还支持一下参数来实现远程的访问：
 - `--port` 设定端口
 - `--model_pb` 指定 ONNX 格式的模型文件
 
+VisualDL的图形系统采用` GraphViz `来可视化ONNX格式模型。
+请安装 [GraphViz](https://www.graphviz.org/download/)确保VisualDL图形系统可以启动
 
 ### 贡献
 

README.md

@@ -142,6 +142,9 @@ Board also supports the parameters below for remote access:
 - `--port` set port
 - `--model_pb` specify ONNX format for model file
 
+The VisualDL Graphing system uses `GraphViz` to visualize the ONNX model. To enable the VisualDL Graph feature,
+please install [GraphViz](https://www.graphviz.org/download/)
+
 ### How to install from pypi
 ```
 pip install --upgrade visualdl

docs/quick_start_cn.md

@@ -89,3 +89,8 @@ visualDL --logdir somedir --model_pb <path_to_model>
 <p align=center>
     <img width="70%" src="https://raw.githubusercontent.com/PaddlePaddle/VisualDL/develop/demo/mxnet/mxnet_graph.gif" />
 </p>
+
+请参阅 [ONNX教程](https://github.com/onnx/tutorials)如何出口ONNX格式模型。
+
+VisualDL的图形系统采用` GraphViz `来可视化ONNX格式模型。
+请安装 [GraphViz](https://www.graphviz.org/download/)确保VisualDL图形系统可以启动

docs/quick_start_en.md

@@ -97,3 +97,8 @@ For example, for the MNIST dataset, Graph component can render model graph as be
 <p align=center>
 <img width="70%" src="https://raw.githubusercontent.com/PaddlePaddle/VisualDL/develop/demo/mxnet/mxnet_graph.gif" />
 </p>
+
+Please consult [ONNX tutorials](https://github.com/onnx/tutorials) on how to export the ONNX format model.
+
+The VisualDL Graphing system uses `GraphViz` to visualize the ONNX model. To enable the VisualDL Graph feature,
+please install [GraphViz](https://www.graphviz.org/download/).