[release-branch.go1.7] doc: document go1.7

Change-Id: Ieae5831b35768a625bf735a38f3d938f23f0b77b
Reviewed-on: https://go-review.googlesource.com/27057Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/27070Reviewed-by: Chris Broadfoot <cbro@golang.org>

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="go1.7">go1.7 (released 2016/08/15)</h2>
+
+<p>
+Go 1.7 is a major release of Go.
+Read the <a href="/doc/go1.7">Go 1.7 Release Notes</a> for more information.
+</p>
+
 <h2 id="go1.6">go1.6 (released 2016/02/17)</h2>
 
 <p>