[css-2020] Add css-sizing-3 to limited experience set per WG resolution. w3c#4715

Author: fantasai

css-2020/Overview.bs

@@ -300,6 +300,15 @@ Fairly Stable Modules with limited implementation experience</h3>
 	We hope to incorporate them into the [[#css|official definition of CSS]] in a future snapshot.
 
 	<dl>
+		<dt><a href="https://www.w3.org/TR/css-sizing-3/">CSS Box Sizing Level 3</a>
+		  [[!CSS-SIZING-3]]
+		<dd>
+			Overlays and extends CSS&sect;10.,
+			expanding the value set of the sizing properties,
+			introducing more precise sizing terminology,
+			and defining with more precision and detail
+			various automatic sizing concepts only vaguely defined in CSS2.
+
 		<dt><a href="https://www.w3.org/TR/css-counter-styles-3/">CSS Counter Styles Level 3</a>
 		  [[CSS-COUNTER-STYLES-3]]
 		<dd>