Added issue for multi color border syntax

Author: LeaVerou

css-backgrounds-4/Overview.bs

@@ -252,6 +252,8 @@ Painting Area: the 'background-clip' property</h3>
 		<img src="images/multicolor-border-dotted.png" alt="">
 	</div>
 
+	Issue: The syntax for comma-separated multiple colors <a href="https://github.com/w3c/csswg-drafts/issues/1172#issuecomment-379878579">is still under active discussion</a> in the CSS WG.
+
 <h2 id="corners">
 Corners</h2>