Add comment to MainWindow with tutorial link

SpritePacker-GUI/MainWindow.cs

@@ -12,6 +12,10 @@ using System.Text;
 
 namespace SpritePacker.GUI
 {
+	/// <summary>
+	/// The main SpritePacker GUI window.
+	/// Uses Gtk#. This tutorial is really helpful: http://zetcode.com/gui/gtksharp/
+	/// </summary>
 	public class MainWindow : Window
 	{
 		private static bool initialised = false;