add sample message to the doc

README.md

@@ -88,9 +88,111 @@ $executor->execute(
 
 ### Iteration callback to get runtime information with a message queue
 
+#### Sample Message for parent / children communication
+
+```{.php}
+<?php
+
+namespace My\Project;
+
+use Saxulum\MessageQueue\MessageInterface;
+
+class SampleMessage implements MessageInterface
+{
+    /**
+     * @var string
+     */
+    private $context;
+
+    /**
+     * @var string
+     */
+    private $message;
+
+    /**
+     * @param string $context
+     * @param string $message
+     */
+    public function __construct(string $context, string $message)
+    {
+        $this->context = $context;
+        $this->message = $message;
+    }
+
+    /**
+     * @param string $json
+     *
+     * @return MessageInterface
+     */
+    public static function fromJson(string $json): MessageInterface
+    {
+        $rawMessage = json_decode($json);
+
+        return new self($rawMessage->context, $rawMessage->message);
+    }
+
+    /**
+     * @return string
+     */
+    public function toJson(): string
+    {
+        return json_encode([
+            'context' => $this->context,
+            'message' => $this->message,
+        ]);
+    }
+
+    /**
+     * @return string
+     */
+    public function getContext(): string
+    {
+        return $this->context;
+    }
+
+    /**
+     * @return string
+     */
+    public function getMessage(): string
+    {
+        return $this->message;
+    }
+}
+```
+
+#### Sample child command
+
+```{.php}
+#!/usr/bin/env php
+<?php
+
+use My\Project\SampleMessage;
+use Saxulum\MessageQueue\SystemV\SystemVSend;
+
+if (!isset($argv[1])) {
+    throw new \InvalidArgumentException('Missing key for SystemVSend');
+}
+
+if (!isset($argv[2])) {
+    throw new \InvalidArgumentException('Missing child id');
+}
+
+$send = new SystemVSend($argv[1]);
+
+for ($i = 0; $i < 100; ++$i) {
+    $message = new SampleMessage($argv[2], sprintf('message %d', $i));
+    $send->send($message);
+
+    echo $message->toJson().PHP_EOL;
+}
+```
+
+#### Parent command
+
 ```{.php}
 <?php
 
+use My\Project\SampleMessage;
 use Symfony\Component\Process\Process;
 use Saxulum\MessageQueue\SystemV\SystemVReceive;
 use Saxulum\ProcessesExecutor\ProcessesExecutor;
@@ -105,7 +207,7 @@ $processes = [
 
 $messages = [];
 
-$receiver = new SystemVReceive(<MessageInterface::class>, 1);
+$receiver = new SystemVReceive(SampleMessage::class, 1);
 
 // make sure the queue is empty
 while (null !== $message = $receiver->receive()) {}