[css-2015] Add Florian as an co-editor

Per the resolution recorded in:
https://lists.w3.org/Archives/Public/www-style/2015Jul/0349.html

Author: frivoal

css-2015/Overview.bs

@@ -9,6 +9,7 @@ URL: https://drafts.csswg.org/css-2015/
 TR: https://www.w3.org/TR/CSS
 Editor: Tab Atkins Jr., Google, http://xanthir.com/
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact
+Editor: Florian Rivoal, Invited Expert, florian@rivoal.net, http://florian.rivoal.net
 Abstract: This document collects together into one definition all the specs that
 	together form the current state of Cascading Style Sheets (CSS) as of 2015.
 	The primary audience is CSS implementers, not CSS authors, as this definition