add chapter about data repositories, and start work on perfomance chapter

implementation/15-adding-content/src/Repository/MarkdownPageRepo.php

@@ -0,0 +1,19 @@
+<?php declare(strict_types=1);
+
+namespace Lubian\NoFramework\Repository;
+
+use Lubian\NoFramework\Model\MarkdownPage;
+
+interface MarkdownPageRepo
+{
+    /**
+     * @return MarkdownPage[]
+     */
+    public function all(): array;
+
+    public function byId(int $id): MarkdownPage;
+
+    public function byTitle(string $title): MarkdownPage;
+
+    public function save(MarkdownPage $page): MarkdownPage;
+}