ipv6: add missing BUGS section for Windows

Change-Id: I08148b1a7fa5f57bcf71f4e49a698360d33a6f8e
Reviewed-on: https://go-review.googlesource.com/34872
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ipv6/icmp.go

@@ -6,6 +6,9 @@ package ipv6
 
 import "golang.org/x/net/internal/iana"
 
+// BUG(mikio): On Windows, methods related to ICMPFilter are not
+// implemented.
+
 // An ICMPType represents a type of ICMP message.
 type ICMPType int