Add metatag viewport to enable mobile scaling

This change tells browsers to scale the viewport to the device-width.
That enables mobile platforms to scale to their correct width.

index.html

@@ -8,6 +8,8 @@
     <meta name="author" content="spilymp">
     <meta name="description" content="An simple Icon Builder for Odoo (IBO), which provide the ability to create app icons in the style of the appicons of the ERP system Odoo.">
 
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+
     <link rel="stylesheet" type="text/css" href="css/milligram.min.css">
     <link rel="stylesheet" type="text/css" href="css/style.css">
 
@@ -325,4 +327,4 @@ <h3>Parameter</h3>
     </footer>
 </body>
 
-</html>
+</html>