Added description meta tag

includes/header.php

@@ -5,6 +5,7 @@
   <!-- Required meta tags -->
   <meta charset="utf-8" />
   <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no" />
+  <meta name="description" content="<?php echo htmlentities(DESC); ?>">
   <link rel="stylesheet" href="./css/darkmode.css" />
   <link rel="stylesheet" href="./css/style.css" />
   <!-- Bootstrap CSS -->