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

implementation/15-adding-content/src/Http/Emitter.php

@@ -0,0 +1,10 @@
+<?php declare(strict_types=1);
+
+namespace Lubian\NoFramework\Http;
+
+use Psr\Http\Message\ResponseInterface;
+
+interface Emitter
+{
+    public function emit(ResponseInterface $response, bool $withoutBody = false): void;
+}