[css-counter-styles] Add example for system: extends

Hey Tab,
Can you add an example to
   http://dev.w3.org/csswg/css-counter-styles/#override-system
? It would help a lot with understanding this section.

Here's one I just gave someone, in case it's helpful:

/* numbers with 1) 2) 3) instead of 1. 2. 3. */
@counter-style paren-decimal {
   system: extends decimal;
   suffix: ") ";
}

Thanks~

~fantasai