update cla

CONTRIBUTING.md

 # Contribute Code
 
-To get started, <a href="https://www.clahub.com/agreements/PaddlePaddle/Paddle">sign the Contributor License Agreement</a>.
+You are welcome to contribute to project PaddlePaddle. To contribute to PaddlePaddle, you have to agree with the 
+[PaddlePaddle Contributor License Agreement](https://gist.github.com/wangkuiyi/0c22c7b1bd3bb7eb27d76f85c3a3e329).
 
 We sincerely appreciate your contribution.  This document explains our workflow and work style.