[css-values] Credit @cdoublev for contributions to the css-values specs

Author: fantasai

css-values-4/Overview.bs

@@ -5559,6 +5559,7 @@ Acknowledgments</h2>
 	Secondly, we would like to acknowledge
 	Anthony Frehner,
 	Emilio Cobos Álvarez,
+	Guillaume Lebas,
 	Koji Ishii,
 	Noam Rosenthal,
 	and

css-values-5/Overview.bs

@@ -3229,6 +3229,7 @@ Acknowledgments</h2>
 	of this module.
 
 	Secondly, we would like to acknowledge
+	Guillaume Lebas,
 	L. David Baron,
 	Mike Bremford,
 	and Sebastian Zartner