add source

README.md

@@ -42,7 +42,7 @@ pip install paddle-serving-server # CPU
 pip install paddle-serving-server-gpu # GPU
 ```
 
-You may need to use a domestic mirror source (in China, you can use the Tsinghua mirror source) to speed up the download.
+You may need to use a domestic mirror source (in China, you can use the Tsinghua mirror source, add "-i https://pypi.tuna.tsinghua.edu.cn/simple" to pip command) to speed up the download. 
 
 <h2 align="center">Quick Start Example</h2>
 

README_CN.md

@@ -43,7 +43,7 @@ pip install paddle-serving-server # CPU
 pip install paddle-serving-server-gpu # GPU
 ```
 
-您可能需要使用国内镜像源（例如清华源）来加速下载。
+您可能需要使用国内镜像源（例如清华源, 在pip命令中添加"-i https://pypi.tuna.tsinghua.edu.cn/simple"）来加速下载。
 
 <h2 align="center">快速启动示例</h2>