Added UserRepositoryInterface

src/Repositories/UserRepositoryInterface.php

@@ -0,0 +1,15 @@
+<?php
+namespace League\OAuth2\Server\Repositories;
+
+use League\OAuth2\Server\Entities\Interfaces\UserEntityInterface;
+
+interface UserRepositoryInterface extends RepositoryInterface
+{
+    /**
+     * Get a user
+     * @param string $username
+     * @param string $password
+     * @return UserEntityInterface
+     */
+    public function getByCredentials($username, $password);
+}