[css-color-4] Point to sample premultiplication code

Author: svgeesus

css-color-4/Overview.bs

@@ -4901,6 +4901,11 @@ Interpolating with alpha</h3>
 
 	</div>
 
+	There is sample JavaScript code
+	for alpha premultiplication and un-premultiplication,
+	for both polar and rectangular color spaces,
+	in [[#color-conversion-code]].
+
 
 <h3 id="hue-interpolation">
 Hue interpolation</h3>