chacha20poly1305: clarify the correct attribution for the AMD64 assembly.

Change-Id: I9c247e49a13788408b4fe49676520d608d8c4380
Reviewed-on: https://go-review.googlesource.com/31090
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

chacha20poly1305/chacha20poly1305_amd64.s

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// This assembly file was lovingly hand-crafted by Vlad Krasnov of CloudFlare.
+
 // +build go1.7
 
 #include "textflag.h"