doc: document go1.3.1

LGTM=r
R=r, rsc
CC=golang-codereviews
https://golang.org/cl/126060043

doc/devel/release.html

@@ -20,6 +20,13 @@ Go 1.3 is a major release of Go.
 Read the <a href="/doc/go1.3">Go 1.3 Release Notes</a> for more information.
 </p>
 
+<h3 id="go1.3.minor">Minor revisions</h3>
+
+<p>
+go1.3.1 (released 2014/08/13) includes bug fixes to the compiler and the the <code>runtime</code>, <code>net</code>, and <code>crypto/rsa</code> packages.
+See the <a href="//code.google.com/p/go/source/list?name=release-branch.go1.3&r=40272ab1339ab2fb9e7160483e5e5d42d6b7e810">change history</a> for details.
+</p>
+
 <h2 id="go1.2">go1.2 (released 2013/12/01)</h2>
 
 <p>