Add a bundle configuration example

Fixes #2 (closed)

src/DependencyInjection/Configuration.php

+<?php
+
+declare(strict_types=1);
+
+namespace DBP\API\StarterBundle\DependencyInjection;
+
+use Symfony\Component\Config\Definition\Builder\TreeBuilder;
+use Symfony\Component\Config\Definition\ConfigurationInterface;
+
+class Configuration implements ConfigurationInterface
+{
+    public function getConfigTreeBuilder()
+    {
+        $treeBuilder = new TreeBuilder('dbp_starter');
+
+        $treeBuilder->getRootNode()
+            ->children()
+            ->scalarNode('secret_token')->end()
+            ->end()
+            ->end();
+
+        return $treeBuilder;
+    }
+}