icmp: add missing BUGS section

Change-Id: If9192dabf9b5cf0d7fefb36dd6d1c1cd8966348a
Reviewed-on: https://go-review.googlesource.com/34434
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

icmp/message.go

@@ -24,6 +24,8 @@ import (
 	"golang.org/x/net/ipv6"
 )
 
+// BUG(mikio): This package is not implemented on NaCl and Plan 9.
+
 var (
 	errMessageTooShort  = errors.New("message too short")
 	errHeaderTooShort   = errors.New("header too short")