doc: mention darwin/arm64 port in go1.5.txt

Change-Id: I3b3f80791a1db4c2b7318f81a115972cd2237f08
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/8787Reviewed-by: David Crawshaw <crawshaw@golang.org>

doc/go1.5.txt

@@ -10,6 +10,7 @@ Go 1.4 required to build (https://golang.org/cl/2470, https://golang.org/cl/2993
 
 New Ports:
 darwin/arm, a.k.a iOS. (https://golang.org/cl/2118, 2119, 3273, 2121, 2122, ..., 2127)
+darwin/arm64
 linux/arm64 (cgo is supported, but only with external linking)
 
 Removed Ports: