[css-ui-4] Carry over and adapt ack prose from level 3

Author: frivoal

css-ui-4/Overview.bs

@@ -771,6 +771,16 @@ input[type=checkbox]:checked {
 
 This appendix is <em>informative</em>.
 
+This specification builds upon [[CSS3-UI]],
+which was edited and written for the most part
+by Tantek Çelik from 1999 to the present,
+first while representing Microsoft, then as an invited expert,
+and most recently while representing Mozilla.
+
+Later work on level 3 and on this level 4
+was done by Florian Rivoal,
+working on behalf of Bloomberg.
+
 	Issue: Add final level 3 content
 
 <h2 class="no-num" id="changes">Appendix B. Changes</h2>