[css-align] Update Changes list.

Author: fantasai

css-align/Overview.bs

@@ -1951,7 +1951,14 @@ Aligning Boxes by Baseline</h3>
 <h2 id="changes">
 Changes</h2>
 
-	Changes since the previous Working Draft include:
+	Changes since the <a href="https://www.w3.org/TR/2017/WD-css-align-3-20170720/#changes">20 July 2017 Working Draft</a> include:
+	<ul>
+		<li>Added the 'row-gap' and 'column-gap' properties and 'gap' shorthand,
+			applying them to both <a>grid layout</a> and <a>flex layout</a>
+			to replace the grid-specific 'grid-row-gap'/'grid-column-gap'/'grid-gap' properties.
+	</ul>
+
+	Changes since the <a href="https://www.w3.org/TR/2017/WD-css-align-3-20170515/">15 May 2017 Working Draft</a> include:
 	<ul>
 		<li>Dropped the ''left'' and ''right'' keywords from 'align-self' and 'align-content'.
 			They may be re-introduced in a later level if there is demand.