[css-pseudo-4] Add change log

Author: schenney-chromium

css-pseudo-4/Overview.bs

@@ -1511,6 +1511,9 @@ Changes</h2>
       and {{CSSPseudoElement/pseudo()}} method
       to address [=sub-pseudo-elements=].
       (<a href="https://github.com/w3c/csswg-drafts/issues/3836">Issue 3836</a>)
+    <li>Defined the behavior when custom property values are not found in the
+      highlight cascade; they are taken from the root.
+      (<a href="https://github.com/w3c/csswg-drafts/issues/6641">Issue 6641</a>)
   </ul>
 
   Significant changes since the <a href="https://www.w3.org/TR/2019/WD-css-pseudo-4-20190225/">25 February 2019 Working Draft</a> include: