[css-color] Add rebeccapurple to the changes list.

Author: tabatkins

css-color/Overview.bs

@@ -4014,4 +4014,6 @@ Changes from Colors 3</h3>
 		<li>''color()'' function, for manipulating colors.
 
 		<li><<named-hue>>, for specifying hsl/hwb hue with readable names, rather than a more opaque angle.
+
+		<li>Addition of named color ''rebeccapurple''.
 	</ol>