Supplement the missing parts of runnable code (#450)

README.md

@@ -119,10 +119,10 @@ Callbacks registered on the Revolt event-loop are automatically run as coroutine
 ```php
 <?php
 
-use function Amp\delay;
-
 require __DIR__ . '/vendor/autoload.php';
 
+use function Amp\delay;
+
 Amp\async(function () {
     print '++ Executing callback passed to async()' . PHP_EOL;
 
@@ -195,6 +195,8 @@ retrieve multiple HTTP resources concurrently:
 ```php
 <?php
 
+require __DIR__ . '/vendor/autoload.php';
+
 use Amp\Future;
 use Amp\Http\Client\HttpClientBuilder;
 use Amp\Http\Client\Request;
@@ -284,6 +286,9 @@ associated `Future` to its caller.
 ```php
 <?php // Example async producer using DeferredFuture
 
+require __DIR__ . '/vendor/autoload.php';
+
+use Amp\Future;
 use Revolt\EventLoop;
 
 function asyncMultiply(int $x, int $y): Future