New Skybox for Episode 1 addon

Just a simple edit of Episode 1's skybox from Doom I to make it more realistic. I DO NOT KNOW IF THIS WORKS WITH SOURCE PORTS OTHER THAN GZDOOM. To install, simply extract and place the WAD the file to whatever folder you choose, and then load it into GZDoom, however you choose to do so. Whether that be by just simply drag & dropping it over GZDoom, or loading it in the ZDL launcher.