Add pubsub example

Fixes #85.

examples/pubsub.php

@@ -0,0 +1,26 @@
+<?php declare(strict_types=1);
+
+require __DIR__ . '/../vendor/autoload.php';
+
+use Amp\Redis\RedisSubscriber;
+use function Amp\async;
+use function Amp\delay;
+use function Amp\Redis\createRedisClient;
+use function Amp\Redis\createRedisConnector;
+
+$client = createRedisClient('redis://');
+$subscriber = new RedisSubscriber(createRedisConnector('redis://'));
+
+$subscription = $subscriber->subscribe('amphp');
+
+async(function () use ($client) {
+    delay(3); // wait for subscription to be active
+
+    $client->publish('amphp', 'New release is out!');
+});
+
+foreach ($subscription as $message) {
+    var_dump($message);
+
+    break;
+}