Add example of how to use the server to send return codes

[doug-co]

I've been playing with the examples, and from looking at the readme, it isn't clear how I would return a not found or server error return code to a request.

I think it would be helpful to have an example.

Thanks!

[doug-co]

With a little more study of the code, my solution is as follows:

replace:

%{record | anlist: [resource]}

with:

case rcode do
  0 -> %{record | anlist: [resource]}
  _ -> %{record | header: %{ record.header | rcode: rcode }}
end

rcode is set at some point in the handle function to based on the server's response to the query.

If you think this would be helpful to others, please add to the README.md

[tungd]

Hi, this is just basic record update, I think people who are familiar with Elixir can figure this out themselves.