[css-grid-1] Update Changes list.

Author: fantasai

css-grid-1/Overview.bs

@@ -4655,6 +4655,13 @@ Major Changes</h4>
 				in a <a>grid area</a>
 				whose edges coincide with the <del>padding</del> <ins>content</ins> edges of the <a>grid container</a>.
 			</blockquote>
+
+		<li id="change-2018-scrollable-grid">
+			The grid (and surrounding padding) is included in the scrollable overflow area.
+			See [[#overflow]].
+			(<a href="https://github.com/w3c/csswg-drafts/issues/3638">Issue 3638</a>,
+			 <a href="https://github.com/w3c/csswg-drafts/issues/3665">Issue 3665</a>)
+
 	</ul>
 
 <h4 id="clarify-2017">