add developer's fluid guide

doc/fluid/getstarted/Developer's_Guide_to_Paddle_Fluid.md

@@ -86,7 +86,7 @@
 <br>
 
 <p align="center">
-  <img src="https://raw.githubusercontent.com/PaddlePaddle/Paddle/develop/doc/fluid/images/fluid_compiler.png" width=100%>
+  <img src="https://raw.githubusercontent.com/PaddlePaddle/Paddle/develop/doc/fluid/images/fluid-compiler.png" width=100%>
 </p>
 
 ---

doc/fluid/getstarted/index_cn.rst

@@ -17,3 +17,4 @@
   :maxdepth: 1
 
   concepts/use_concepts_cn.rst
+  developer's_guide_to_paddle_fluid.md

doc/fluid/getstarted/index_en.rst

@@ -16,3 +16,4 @@ Here is an example of linear regression. It introduces workflow of PaddlePaddle,
   :maxdepth: 1
 
   concepts/index_en.rst
+  developer's_guide_to_paddle_fluid.md