doc: document Go 1.6

Change-Id: I4910105d48ed650289ecb1490d556929db05bc38
Reviewed-on: https://go-review.googlesource.com/19526Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-on: https://go-review.googlesource.com/19527

doc/devel/release.html

@@ -30,6 +30,13 @@ to fix critical security problems in both Go 1.4 and Go 1.5 as they arise.
 See the <a href="/security">security policy</a> for more details.
 </p>
 
+<h2 id="go1.6">go1.6 (released 2015/02/17)</h2>
+
+<p>
+Go 1.6 is a major release of Go.
+Read the <a href="/doc/go1.6">Go 1.6 Release Notes</a> for more information.
+</p>
+
 <h2 id="go1.5">go1.5 (released 2015/08/19)</h2>
 
 <p>