Add README documentation to ExtensionPage (#3094)

Co-authored-by: Alexander Skvortsov <sasha.skvortsov109@gmail.com>

src/Api/Controller/ShowExtensionReadmeController.php

@@ -0,0 +1,47 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+namespace Flarum\Api\Controller;
+
+use Flarum\Api\Serializer\ExtensionReadmeSerializer;
+use Flarum\Extension\ExtensionManager;
+use Flarum\Http\RequestUtil;
+use Illuminate\Support\Arr;
+use Psr\Http\Message\ServerRequestInterface;
+use Tobscure\JsonApi\Document;
+
+class ShowExtensionReadmeController extends AbstractShowController
+{
+    /**
+     * @var ExtensionManager
+     */
+    protected $extensions;
+
+    /**
+     * {@inheritdoc}
+     */
+    public $serializer = ExtensionReadmeSerializer::class;
+
+    public function __construct(ExtensionManager $extensions)
+    {
+        $this->extensions = $extensions;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    protected function data(ServerRequestInterface $request, Document $document)
+    {
+        $extensionName = Arr::get($request->getQueryParams(), 'name');
+
+        RequestUtil::getActor($request)->assertAdmin();
+
+        return $this->extensions->getExtension($extensionName);
+    }
+}