Add installation instructions & a minimal autoloader

Author: jkphl

README.md

@@ -129,7 +129,13 @@ The `Generator` will try to calculate the anticipated image sizes for the regist
 
 ## Installation
 
-This library requires PHP 7.1 or later. I recommend using the latest available version of PHP as a matter of principle. It has no userland dependencies.
+This library requires PHP 7.1 or later. I recommend using the latest available version of PHP as a matter of principle. It has no userland dependencies. It's installable and autoloadable via [Composer](https://getcomposer.org/) as [jkphl/responsive-images-css](https://packagist.org/packages/jkphl/responsive-images-css).
+
+```bash
+composer require jkphl/responsive-images-css
+```
+
+Alternatively, [download a release](https://github.com/jkphl/responsive-images-css/releases) or clone this repository, then require or include its [`autoload.php`](autoload.php) file.
 
 ## Dependencies
 

autoload.php

@@ -0,0 +1,66 @@
+<?php
+
+/**
+ * responsive-images-css
+ *
+ * @category  Jkphl
+ * @package   Jkphl\Respimgcss
+ * @author    Joschi Kuphal <[email protected]> / @jkphl
+ * @copyright Copyright © 2018 Joschi Kuphal <[email protected]> / @jkphl
+ * @license   http://opensource.org/licenses/MIT The MIT License (MIT)
+ */
+
+/***********************************************************************************
+ *  The MIT License (MIT)
+ *
+ *  Copyright © 2018 Joschi Kuphal <[email protected]> / @jkphl
+ *
+ *  Permission is hereby granted, free of charge, to any person obtaining a copy of
+ *  this software and associated documentation files (the "Software"), to deal in
+ *  the Software without restriction, including without limitation the rights to
+ *  use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ *  the Software, and to permit persons to whom the Software is furnished to do so,
+ *  subject to the following conditions:
+ *
+ *  The above copyright notice and this permission notice shall be included in all
+ *  copies or substantial portions of the Software.
+ *
+ *  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ *  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ *  FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ *  COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ *  IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ *  CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ ***********************************************************************************/
+
+/**
+ * Register a minimal PSR-4 compatible autoloader
+ *
+ * @see http://www.php-fig.org/psr/psr-4/
+ */
+spl_autoload_register(
+    function ($class) {
+        $namespace = 'Jkphl';
+        $prefixes  = [
+            "{$namespace}\\" => [
+                __DIR__.'/src',
+            ],
+        ];
+        foreach ($prefixes as $prefix => $dirs) {
+            $prefixLength = strlen($prefix);
+            if (substr($class, 0, $prefixLength) !== $prefix) {
+                continue;
+            }
+            $class = substr($class, $prefixLength);
+            $part  = str_replace('\\', DIRECTORY_SEPARATOR, $class).'.php';
+            foreach ($dirs as $dir) {
+                $dir  = str_replace('/', DIRECTORY_SEPARATOR, $dir);
+                $file = $dir.DIRECTORY_SEPARATOR.$part;
+                if (is_readable($file)) {
+                    require $file;
+                    return;
+                }
+            }
+        }
+    }
+);