[release-branch.go1.7] doc: document go1.7.4 and go1.6.4

Change-Id: I0728afe6a1d1e0aee4701e51a5548fa9fd637b66
Reviewed-on: https://go-review.googlesource.com/33795Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/33796Reviewed-by: Chris Broadfoot <cbro@golang.org>

doc/devel/release.html

@@ -63,6 +63,12 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.7.3">Go
 1.7.3 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.7.4 (released 2016/12/01) includes two security fixes.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.7.4">Go
+1.7.4 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.6">go1.6 (released 2016/02/17)</h2>
 
 <p>
@@ -94,6 +100,13 @@ See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.6.3">Go
 1.6.3 milestone</a> on our issue tracker for details.
 </p>
 
+<p>
+go1.6.4 (released 2016/12/01) includes two security fixes.
+It contains the same fixes as Go 1.7.4 and was released at the same time.
+See the <a href="https://github.com/golang/go/issues?q=milestone%3AGo1.7.4">Go
+1.7.4 milestone</a> on our issue tracker for details.
+</p>
+
 <h2 id="go1.5">go1.5 (released 2015/08/19)</h2>
 
 <p>