Add an wizard callback listener for the radius field.

Fix readme markup.

Add missing wizard class.

src/DependencyInjection/LeafletGeocodeWidgetExtension.php

@@ -0,0 +1,37 @@
+<?php
+
+/**
+ * Geocode backend widget based on Leaflet.
+ *
+ * @package    netzmacht
+ * @author     David Molineus <david.molineus@netzmacht.de>
+ * @copyright  2016-2018 netzmacht David Molineus. All rights reserved.
+ * @license    LGPL-3.0 https://github.com/netzmacht/contao-leaflet-geocode-widget/blob/master/LICENSE
+ * @filesource
+ */
+
+namespace Netzmacht\Contao\Leaflet\GeocodeWidget\DependencyInjection;
+
+use Symfony\Component\Config\FileLocator;
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\DependencyInjection\Extension\Extension;
+use Symfony\Component\DependencyInjection\Loader\YamlFileLoader;
+
+/**
+ * Class LeafletGeocodeWidgetExtension
+ */
+class LeafletGeocodeWidgetExtension extends Extension
+{
+    /**
+     * {@inheritdoc}
+     */
+    public function load(array $configs, ContainerBuilder $container)
+    {
+        $loader = new YamlFileLoader(
+            $container,
+            new FileLocator(dirname(__DIR__) . '/Resources/config')
+        );
+
+        $loader->load('listeners.yml');
+    }
+}