[css-ui-4] Add previous version

Author: frivoal

css-ui-4/Overview.bs

@@ -19,6 +19,7 @@ Title: CSS Basic User Interface Module Level 4
 ED: https://drafts.csswg.org/css-ui-4/
 Status Text: This specification will include and extend <cite>CSS Basic User Interface Module Level&nbsp;3.</cite> [[CSS-UI-3]]
 TR: https://www.w3.org/TR/css-ui-4/
+Previous Version: https://www.w3.org/TR/2017/WD-css-ui-4-20171222/
 Previous Version: https://www.w3.org/TR/2015/WD-css-ui-4-20150922/
 Shortname: css-ui
 Level: 4