Add abstract class 'UserManager'

src/UserManager.php

@@ -0,0 +1,12 @@
+<?php
+
+/*
+ * PHP-Auth (https://github.com/delight-im/PHP-Auth)
+ * Copyright (c) delight.im (https://www.delight.im/)
+ * Licensed under the MIT License (https://opensource.org/licenses/MIT)
+ */
+
+namespace Delight\Auth;
+
+/** Abstract base class for components implementing user management */
+abstract class UserManager {}