[css-scrollbars] Add myself as coeditor

Resolution at: https://lists.w3.org/Archives/Public/www-style/2020Feb/0011.html

Author: frivoal

css-scrollbars-1/Overview.bs

@@ -9,6 +9,7 @@ ED: https://drafts.csswg.org/css-scrollbars/
 Repository: https://github.com/w3c/csswg-drafts/tree/master/css-scrollbars-1 css-scrollbars
 Editor: Tantek Çelik, Mozilla https://www.mozilla.org/, http://tantek.com/contact, w3cid 1464
 Editor: Rossen Atanassov, Microsoft, ratan@microsoft.com, w3cid 49885
+Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net/, w3cid 43241
 Abstract: CSS Scrollbars standardizes the ability to style scrollbar controls, such as their color,
           introduced in 2000 by Windows IE 5.5. 
           This is useful when building web applications which use color schemes very different from