doc: add crypto/sha512 additions to go1.5.html

Change-Id: I805bb7ba40c8ed579c4ca796e408995586d219b3
Reviewed-on: https://go-review.googlesource.com/12434Reviewed-by: Andrew Gerrand <adg@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.html

@@ -830,6 +830,12 @@ now verifies that the point is actually on the curve.
 This change guards against certain attacks.
 </li>
 
+<li>
+The <a href="/pkg/crypto/sha512/"><code>crypto/sha512</code></a>
+package now has support for the two truncated versions of
+the SHA-512 hash algorithm, SHA-512/224 and SHA-512/256.
+</li>
+
 <li>
 The <a href="/pkg/crypto/tls/"><code>crypto/tls</code></a> package
 minimum protocol version now defaults to TLS 1.0.