Doc update

[delitescere]

Thank you for an awesome Zipkin tool for Elixir!

Using Phoenix, I had a little problem with the request_id not being found, it seems like now one has to explicitly use the Plug.RequestId in one's endpoint.ex (for example). Mine looks like this:

defmodule Ops.Web.Endpoint do
.
.
.
  # Zipkin (needs a `x-request-id` header, generated by the RequestId plug)
  plug Plug.RequestId
  plug PhxExRay.Tracing.Plug
.
.
.

Otherwise, it will be nil and all the subsequent code expects the value to be a binary.

I thought a small update in the readme might help others in the future.

Many thanks again,
Josh