Add docs badge to README

I just added the badge you requested to your README: ![Inline docs](http://inch-pages.github.io/github/voanhduy1512/hello_sign.png)

Your status page for this project is http://inch-pages.github.io/github/voanhduy1512/hello_sign

If you have any questions, feel free to post them here or open an issue in the Inch Pages project.

Thanks for giving this a shot!