add centos6 info to compile doc

doc/COMPILE.md

@@ -36,6 +36,8 @@ cd Serving && git submodule update --init --recursive
 export PYTHONROOT=/usr/
 ```
 
+In the default centos7 image we provide, the Python path is `/usr/bin/python`. If you want to use our centos6 image, you need to set it to `export PYTHONROOT=/usr/local/python2.7/`.
+
 ## Compile Server
 
 ### Integrated CPU version paddle inference library

doc/COMPILE_CN.md

@@ -36,6 +36,8 @@ cd Serving && git submodule update --init --recursive
 export PYTHONROOT=/usr/
 ```
 
+我们提供默认Centos7的Python路径为`/usr/bin/python`，如果您要使用我们的Centos6镜像，需要将其设置为`export PYTHONROOT=/usr/local/python2.7/`。
+
 ## 编译Server部分
 
 ### 集成CPU版本Paddle Inference Library