Create CONTRIBUTING_CN.md

CONTRIBUTING_CN.md

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+如果您想要对 [xkcptun](https://github.com/liudf0716/xkcptun)项目做贡献, 请参考如下步骤和规则:
+
+1. fork本项目先:
+
+    ![fork](http://oi58.tinypic.com/jj2trm.jpg)
+
+2. Clone 您fork完的项目:
+
+    ```
+    git clone git@github.com:your_github_username/xkcptun.git
+    ```
+
+3. clone后请创建一个branch分支:
+
+    ```
+    git checkout -b "xkcptun-1-fix"
+    ```
+    名称请任意定
+
+4. 在该分支下修改您要提交的代码（提交前请测试通过）
+
+5. 完成修改后不要忘记在 [contributor](https://github.com/liudf0716/xkcptun/blob/master/contributors.md)中添加您的大名.
+
+6. Commit and push 您的更改, 在github上创建一个pull request.
+
+    git commit --signoff  
+    git push -f
+    
+7. Awaiting review, if accepted, merged!
+
+
+
+**IMPORTANT**
+
+切记，不要忘记更新您fork的xkcptun项目
+
+Please, don't forget to update your fork. While you made your changes, the content of the `master` branch can change because other pull requests were merged and it can create conflicts. This is why you have to rebase on `master` every time before pushing your changes and check that your branch doesn't have any conflicts with `master`.