Autoloader for manual installation

Added PSR4 Autoloader
Updated README with manual installation instructions

src/Spout/Autoloader/autoload.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Box\Spout\Autoloader;
+
+require_once 'Psr4Autoloader.php';
+
+/**
+ * @var string $srcBaseDirectory
+ * Full path to "src/Spout" which is what we want "Box\Spout" to map to.
+ */
+$srcBaseDirectory = dirname(dirname(__FILE__));
+
+$loader = new Psr4Autoloader();
+$loader->register();
+$loader->addNamespace('Box\Spout', $srcBaseDirectory);