Add changes entry

Author: kbabbitt

css-mixins-1/Overview.bs

@@ -2725,4 +2725,6 @@ Changes {#changes}
 
 Changes since the <a href="https://www.w3.org/TR/2025/WD-css-mixins-1-20250515/">First Public Working Draft of 15 May 2025</a>:
 
+* Made parentheses optional for mixins without parameters.
+	(<a href="https://github.com/w3c/csswg-drafts/issues/13015">Issue 13015</a>)
 * Added Web Platform Tests coverage