improve and perfect document

Signed-off-by: Jianhui Zhao <jianhuizhao329@gmail.com>

CONTRIBUTING.md

@@ -0,0 +1,40 @@
+Contributing
+================================================================================
+
+If you want to contribute to [rttys](https://github.com/zhaojh329/rttys), please follow these simple rules:
+
+1. Press the fork button:
+
+    ![fork](http://oi58.tinypic.com/jj2trm.jpg)
+
+2. Clone the repository from your account with:
+
+    ```
+    git clone https://github.com/your_github_username/rttys.git
+    ```
+
+3. Create a new branch with:
+
+    ```
+    git checkout -b "rttys-1-fix"
+    ```
+    You can name it however you want.
+
+4. Make your changes
+
+5. Commit and push your changes, then make a pull request from Github.
+
+    git commit --signoff
+    git push
+ 
+6. Awaiting review, if accepted, merged!
+
+**IMPORTANT**
+
+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`.
+
+Thank you.