Add usage manual for VisualDL (#913)

* Add usage manual for VisualDL
The usage manual will be helpful for beginners to make use of VisualDL.

Components of VisualDL are introduced in details, including:
	* Definition of Functions
	* Parameters explanation
	* Demo program for each component

* add_index

doc/fluid/user_guides/howto/evaluation_and_debugging/debug/index.rst

-############
-Debug 工具
-############
-
-PaddlePaddle 提供了如下方式方便 Debug 训练 情况
-
-.. toctree::
-   :maxdepth: 2
-
-   visualdl.md

doc/fluid/user_guides/howto/evaluation_and_debugging/debug/index_cn.rst

+.. PaddlePaddle Fluid documentation master file, created by
+   sphinx-quickstart on Thu Jun  7 17:04:53 2018.
+   You can adapt this file completely to your liking, but it should at least
+   contain the root `toctree` directive.
+
+##############
+VisualDL 工具
+##############
+
+..  toctree::
+    :maxdepth: 1
+
+
+    visualdl.md
+    visualdl_usage.md
+

doc/fluid/user_guides/howto/evaluation_and_debugging/debug/index_en.rst

+VisualDL Tools
+==========================
+
+..  toctree::
+    :maxdepth: 1
+
+    visualdl_en.md
+    visualdl_usage_en.md

doc/fluid/user_guides/howto/evaluation_and_debugging/debug/visualdl.md

-# Visual DL工具
+# VisualDL 工具简介
 <p align="center">
   <img src="https://raw.githubusercontent.com/PaddlePaddle/VisualDL/develop/docs/images/vs-logo.png" width="60%" />
 </p>

doc/fluid/user_guides/howto/evaluation_and_debugging/debug/visualdl_en.md

-# Visual DL Toolset
+# Introduction to VisualDL Toolset
 
 <p align="center">
   <img src="https://raw.githubusercontent.com/PaddlePaddle/VisualDL/develop/docs/images/vs-logo.png" width="60%" />
@@ -257,7 +257,7 @@ visualDL also supports following optional parameters:
 
 VisualDL is initially created by [PaddlePaddle](http://www.paddlepaddle.org/) and
 [ECharts](http://echarts.baidu.com/).
-We welcome everyone to use, comment and contribute to Visual DL :)
+We welcome everyone to use, comment and contribute to VisualDL :)
 
 ## More details
 

doc/fluid/user_guides/howto/evaluation_and_debugging/index_cn.rst

@@ -12,4 +12,4 @@
     :hidden:
 
     evaluation/metrics.rst
-    debug/visualdl.md
+    debug/index_cn.rst

doc/fluid/user_guides/howto/evaluation_and_debugging/index_en.rst

@@ -14,4 +14,4 @@ There are two articles in this section:
     :hidden:
 
     evaluation/metrics_en.rst
-    debug/visualdl_en.md
+    debug/index_en.rst