Update CONTRIBUTE.md

doc/CONTRIBUTE.md

-# 贡献指南
+# Contribution Guideline
 
-## 如何贡献代码
+## How to contribute
 
-### 贡献代码
+### Contribute Code
 
-如果您在Paddle Serving上有了改进，请给我们发Pull Requests！Github提供了提交Pull Requests的参考[howto](https://help.github.com/articles/using-pull-requests/)。
+If you have improvements on Paddle Serving, please send us Pull Requests! github.com provides guidelines for submitting pull requests [howto](https://help.github.com/articles/using-pull-requests/).