icmp: add missing BUGS section
icmp: add missing BUGS section

Change-Id: If9192dabf9b5cf0d7fefb36dd6d1c1cd8966348a
Reviewed-on: https://go-review.googlesource.com/34434
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net