The source code is in the jar file that you download. You can open this file with WinRAR or any other unzipping program. Look in the jar file for the folder named "src".
To distribute this with your custom deathquotes.txt file, open the jar file with WinRAR then in that window open the folder "assets". Replace the deathquotes.txt file there with your own. Close the jar file you have open in WinRAR. Important: Please then rename the jar file adding a dash on the end of the file name then your name or a descriptive short name for this change. This will keep your modified jar file from getting mixed up with the originals. Doing this will make your custom deathquotes.txt file be the default one. Any time there is no deathquotes.txt file, yours will be created automatically.