Add interface for commands

src/Commands/CommandContract.php

+<?php
+/**
+ * This file is part of the TelegramBot package.
+ *
+ * (c) Avtandil Kikabidze aka LONGMAN <akalongman@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Longman\TelegramBot\Commands;
+
+/**
+ * Command contract class
+ *
+ * Interface for commands
+ */
+interface CommandContract
+{
+
+}