Add exception marker interface

Author: slavcodev

src/DateRangeException.php

@@ -16,7 +16,7 @@
 /**
  * Date range exception.
  */
-final class DateRangeException extends DomainException
+final class DateRangeException extends DomainException implements Exception
 {
     /**
      * @param string $message

src/Exception.php

@@ -0,0 +1,18 @@
+<?php
+/**
+ * This file is part of Zee Project.
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ *
+ * @see https://github.com/zee/
+ */
+
+namespace Zee\DateRange;
+
+/**
+ * Marker interface.
+ */
+interface Exception
+{
+}