crypto/block: mark as deprecated.

R=rsc
CC=golang-dev
https://golang.org/cl/3909041

src/pkg/crypto/block/cipher.go

@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// The block package is deprecated, use cipher instead.
 // The block package implements standard block cipher modes
 // that can be wrapped around low-level block cipher implementations.
 // See http://csrc.nist.gov/groups/ST/toolkit/BCM/current_modes.html