Add some more notes about SFX

SabreTools.Models/AdvancedInstaller/SFX.cs

@@ -5,6 +5,14 @@ namespace SabreTools.Models.AdvancedInstaller
     /// Advanced Installer SFX file. These SFX files store
     /// all files uncompressed sequentially in the overlay
     /// of an executable.
+    /// 
+    /// The design is similar to the end of central directory
+    /// in a PKZIP file. The footer needs to be read before
+    /// the entry table as both the pointer to the start of
+    /// the table as well as the entry count are included there.
+    /// 
+    /// The layout of this is derived from the layout in the
+    /// physical file.
     /// </summary>
     public class SFX
     {