doc: add Go 1.13 to release history

Change-Id: I3340561c0b17bf28d8d480e00f5bc8afb2a897ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/193041
Run-TryBot: Andrew Bonventre <andybons@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>

doc/contrib.html

@@ -34,6 +34,7 @@ We encourage all Go users to subscribe to
 <p>A <a href="/doc/devel/release.html">summary</a> of the changes between Go releases. Notes for the major releases:</p>
 
 <ul>
+	<li><a href="/doc/go1.13">Go 1.13</a> <small>(September 2019)</small></li>
 	<li><a href="/doc/go1.12">Go 1.12</a> <small>(February 2019)</small></li>
 	<li><a href="/doc/go1.11">Go 1.11</a> <small>(August 2018)</small></li>
 	<li><a href="/doc/go1.10">Go 1.10</a> <small>(February 2018)</small></li>