[css-values-3] Update Changes list

Author: fantasai

css-values-3/Overview.bs

@@ -2155,6 +2155,8 @@ Changes</h2>
 			(<a href="https://drafts.csswg.org/css-values-3/issues-cr-2016#issue-9">Issue 9</a>)
 		<li>Defined that ''0'' parses as <<number>> if it's ambiguous whether it's a <<number>> or a <<length>>.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/489">Issue 489</a>)
+		<li>Defined empty ''url()''s to refer to an invalid URL, rather than resolving to the URL of the style sheet.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/2211">Issue 2211</a>)
 		<li>Removed (unused) ability for percentages to be treated as a <<number>> type in ''calc()''.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/1463">Issue 1463</a>)
 		<li>Clarified that high-resolution screens should anchor on device pixels, not physical units.