Add guide

documentation/guides/publisher/with_default_exchange.md

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+Publisher with default exchange
+===============================
+
+> [The default exchange](https://www.rabbitmq.com/tutorials/amqp-concepts.html#exchange-default) is a direct exchange with no name (empty string) pre-declared by the broker. It has one special property that makes it very useful for simple applications: every queue that is created is automatically bound to it with a routing key which is the same as the queue name.
+
+# Example
+
+~~~elixir
+defmodule WithDefaultExchange do
+  @behaviour GenRMQ.Publisher
+
+  def start_link() do
+    GenRMQ.Publisher.start_link(__MODULE__, name: __MODULE__)
+  end
+
+  def publish_message(message, routing_key) do
+    Logger.info("Publishing message #{inspect(message)}")
+    GenRMQ.Publisher.publish(__MODULE__, message, routing_key)
+  end
+
+  def init() do
+    [
+      exchange: :default,
+      connection: "amqp://guest:guest@localhost:5672"
+    ]
+  end
+end
+~~~