[css-display-3] Add more Previous Versions.

Author: tabatkins

css-display-3/Overview.bs

@@ -7,6 +7,8 @@ Shortname: css-display
 Group: csswg
 Level: 3
 TR: https://www.w3.org/TR/css-display-3/
+Previous Version: https://www.w3.org/TR/2018/WD-css-display-3-20180809/
+Previous Version: https://www.w3.org/TR/2018/WD-css-display-3-20180420/
 Previous Version: https://www.w3.org/TR/2017/WD-css-display-3-20170720/
 Previous Version: https://www.w3.org/TR/2017/WD-css-display-3-20170126/
 Previous Version: https://www.w3.org/TR/2015/WD-css-display-3-20151015/