[css-2018] Add Previous version

Author: fantasai

css-2018/Overview.bs

@@ -7,6 +7,7 @@ Group: CSSWG
 Work Status: revising
 URL: https://drafts.csswg.org/css-2018/
 TR: https://www.w3.org/TR/CSS/
+Previous Version: https://www.w3.org/TR/2018/NOTE-css-2018-20181115/
 Editor: Tab Atkins Jr., Google, http://xanthir.com/, w3cid 42199
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Florian Rivoal, Invited Expert, https://florian.rivoal.net, w3cid 43241