Add pool sharing example

Relates to #367.

examples/pooling/3-pool-share.php

@@ -0,0 +1,33 @@
+<?php declare(strict_types=1);
+
+use Amp\CancelledException;
+use Amp\Http\Client\Connection\UnlimitedConnectionPool;
+use Amp\Http\Client\HttpClientBuilder;
+use Amp\Http\Client\HttpException;
+use Amp\Http\Client\Request;
+
+require __DIR__ . '/../.helper/functions.php';
+
+try {
+    $pool = new UnlimitedConnectionPool();
+
+    $clientA = (new HttpClientBuilder)
+        ->usingPool($pool)
+        ->followRedirects(0) // no follow
+        ->retry(3)
+        ->build();
+
+    $clientB = (new HttpClientBuilder)
+        ->usingPool($pool)
+        ->followRedirects() // follow
+        ->retry(3)
+        ->build();
+
+    $responseA = $clientA->request(new Request($argv[1] ?? 'https://httpbin.org/redirect-to?url=%2f'));
+    $responseB = $clientB->request(new Request($argv[1] ?? 'https://httpbin.org/redirect-to?url=%2f'));
+
+    dumpResponseTrace($responseA);
+    dumpResponseTrace($responseB);
+} catch (HttpException|CancelledException $error) {
+    echo $error;
+}