First div CSS example in Portfolio 1 is confusing

The first Portfolio chapter has people create a div with `width: 100%; margin: 0 auto;` and says that this centres the div.

A lot of people then put content into the div and were asking why it wasn't centered. I think it may be necessary to explain more of the box model at this point, or use a more straightforward (or fleshed out) initial example.
