[css-align] Add previous version

Author: fantasai

css-align/Overview.bs

@@ -10,6 +10,7 @@ ED: https://drafts.csswg.org/css-align/
 TR: https://www.w3.org/TR/css-align-3/
 Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
 Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
+Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170720/
 Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170515/
 Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170407/
 Previous Version: https://www.w3.org/TR/2017/WD-css-align-3-20170215/