doc: document Go 1.5 on release page

This makes sure the release page in the release will mention the release.

Fixes #12102.

Change-Id: I36befd7dba7ba9e70ae3335e21c8841179ac4eff
Reviewed-on: https://go-review.googlesource.com/13490Reviewed-by: Rob Pike <r@golang.org>

doc/devel/release.html

@@ -12,6 +12,13 @@ git pull
 git checkout <i>release-branch</i>
 </pre>
 
+<h2 id="go1.5">go1.5 (released 2015/08/19)</h2>
+
+<p>
+Go 1.5 is a major release of Go.
+Read the <a href="/doc/go1.5">Go 1.5 Release Notes</a> for more information.
+</p>
+
 <h2 id="go1.4">go1.4 (released 2014/12/10)</h2>
 
 <p>