added box model to basic css step page

Author: lumberg

sites/en/frontend/basic_CSS.step

@@ -95,6 +95,11 @@ When you save and refresh your browser, you should see the styles you added:
 <img src='img/hello_style.png'>
 
 The `type` and `media` attributes inside of the `<style>` tag give the browser more context for how to deal with the included information. In this case, we're telling it that the type is HTML/CSS, and that that it's being displayed on a screen (as opposed to `media=\"print\"`, which is usually specially styled so it looks reasonable when printed.
+
+To visualize the relations between elements of Content, Padding, Border, and Margin, otherwise known as the CSS Box Model, here is an example of how this fits in:
+
+<img src='img/boxmodel.png'>
+ 
     MARKDOWN
   end