remove changelog comment

we're using git, so no need for embedded changelog
incorporate suggestions from Brian E Carpenter

- make it more clear that the siit-dc gateways are to be located outside
  of the entire ipv6 application infrastructure (including firewalls,
  load balancers, idses, etc)
- suggest that the siit-dc gateway sets the ipv6 flow label in
  accordance with rfc6437