Add a guide for adding design using HTML & CSS

I am from [Railsgirls Beijing Team](http://railsgirls.com/beijing).
I write a simple guide for the next step of the [basic tutorial](http://guides.railsgirls.com/app/)
If girls followed the guide, they will get the app looks like [Rails Girls App](http://railsgirlsapp.herokuapp.com/ideas)
Feel free to revise my English since It's not my native language.