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

implementation/15-adding-content/src/Service/Time/SystemClockNow.php

@@ -0,0 +1,13 @@
+<?php declare(strict_types=1);
+
+namespace Lubian\NoFramework\Service\Time;
+
+use DateTimeImmutable;
+
+final class SystemClockNow implements Now
+{
+    public function __invoke(): DateTimeImmutable
+    {
+        return new DateTimeImmutable;
+    }
+}