[css-grid-2] Add list of changes since L1

Author: fantasai

css-grid-2/Overview.bs

@@ -5314,6 +5314,16 @@ Changes</h2>
 	See <a href="https://www.w3.org/TR/2019/WD-css-grid-2-20191203/#changes">Changes during Working Draft</a>
 	for previous changes.
 
+<h3 id="changes-2">
+Additions Since Level 2</h3>
+
+	The following features have been added since
+	<a href="http://www.w3.org/TR/css-grid-1/">Level 1</a>:
+
+	<ul>
+		<li>[[#subgrids|Subgrids]] (''subgrid'')
+	</ul>
+
 <h2 id="acknowledgements">
 Acknowledgements</h2>