Improve an answer to the box model question (yangshun#110)

* Improve an answer to the box model question

The question:
> What does `* { box-sizing: border-box; }` do? What are its advantages?
is actually 2 questions and in my opinion only the first one is answered. I tried to answer the second one, plus I used a reference post from Paul Irish.

* Update css-questions.md

questions/css-questions.md

@@ -357,6 +357,11 @@ The box model has the following rules:
 * `box-sizing: border-box` changes how the `width` and `height` of elements are being calculated, `border` and `padding` are also being included in the calculation.
 * The `height` of an element is now calculated by the content's `height` + vertical `padding` + vertical `border` width.
 * The `width` of an element is now calculated by the content's `width` + horizontal `padding` + horizontal `border` width.
+* Taking into account `padding`s and `border`s as part of our box model resonates better with how designers actually imagine content in grids.
+
+###### References
+
+* https://www.paulirish.com/2012/box-sizing-border-box-ftw/
 
 [[↑] Back to top](#css-questions)