doc: mention Unicode update in release notes

Fixes #33472

Change-Id: Iab69e69589f2e017f4cf9770858884b1a570c89e
Reviewed-on: https://go-review.googlesource.com/c/go/+/188799Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.13.html

@@ -992,3 +992,13 @@ godoc
 
 </dl><!-- time -->
 
+<dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
+  <dd>
+    <p>
+      The <a href="/pkg/unicode/"><code>unicode</code></a> package and associated
+      support throughout the system has been upgraded from Unicode 10.0 to
+      <a href="http://www.unicode.org/versions/Unicode11.0.0/">Unicode 11.0</a>,
+      which adds 684 new characters, including seven new scripts, and 66 new emoji.
+    </p>
+
+</dl><!-- unicode -->