[css-color-5][editorial] Mention contrast-color() in intro

Author: svgeesus

css-color-5/Overview.bs

@@ -131,6 +131,7 @@ Introduction {#intro}
 	like yellow and blue, can have the same HSL lightness).
 
 	This module adds the new functions
+	''contrast-color()'', 
 	''color-mix()'' and ''light-dark()'',
 	and extends existing ones with [[#relative-colors|relative color syntax]].