Double Barrel add-on addon

This is an optional add-on file that can be dropped into the d3_gameplay_fixes folder (named d3baselite) that adds the double barrel shotgun on default Doom 3.

When in any sp map, you found an item_backpack, then you get also the double barrel shotgun with it, so this .pk4 file mod add-on can be added or removed optionally with easy.