How to start Doomsday from Steam
Cleanup Note: This article should be cleaned up to be more readable and/or to more closely follow the Wiki article guidelines.
In this tutorial we will show you how to make multiple non-Steam shortcuts in Steam with seems simple enough but do to how steam is it's not also i will be showing you how to set up an RSP file and using the -vdmap command line as if you do not then you will not be able to use your resources you have downloaded like the Heretic resource pack.
Navigate to your doomsday/bin folder and then make 2 copies of doomsday.exe you can rename them if you want witch i did to help me.
If you know how to add non-steam games to steam then skip this if not then here is what to do. Click on add a game then select add a non-steam game then brows to one of the newly copied exe you just made so it's able to be put on steam. click add selectid programs. do the same thing for the other one. the reason we do this is steam can only have 1 exe of the same name loaded and if you try to load it a gen it will default to the first one always so were just making 2 profiles so to speak. Note you have add them both at once here. 
Now you should have 2 new games in your list thats great but we want them to point to doomsday.exe not the copys so right click on them and select properties and change the target to the doomsday.exe if you had lets say named your copy exes doom1 and doom2 then just rename that to dommsday and your good. Note: at this point you could set it up for the game you want with -game and you could also change the name of this new game as it will have the copy exe name.
After finished with steam delete the copy exe you made as they are not needed a gen.
Now lets create an RSP file. you can name it what ever you want. an RSP file is just a text document that dose command line options. now if you only have 3 things to load like wads or pk3's then use the formula you would to load a file with the -file command. here is an example of how one would look. -file F:\doomsday\snowberry\addons\jdmu-all-remix-sycraft-v4.pk3 F:\doomsday\snowberry\addons\dwango5.wad you can also add the -game command before the -file command to load your game that way as well.
Once you have your RSP file made you need to tell your steam shortcuts you made within steam to load the RSP files to do this go back into the properties of then like we did in Step3 but in the target area add @"Directory where you have the RSP file\RSP file and then it should load. an example of this is @F:\doomsday\1.rsp and were done but if you have more addons then 3 then continue reading. An example of what your target area should look like now is "F:\Doomsday\bin\Doomsday.exe" -game doom1-ultimate @F:\doomsday\1.rsp note this assumes you included the -game command.
-Step1E: if you have lets say the JDoom Resource pack that you want to load you can use the -vdmap or Virtual directory mapping to load your addons. to do this useing the RSP file add -vdmap "directory of addons" "directory of auto load folder" an example of this would be -vdmap "F:\DDayaddons\Doom\PK3" "F:\Doomsday\Data\jDoom\Auto" note that if you wanted to do multiples to the same folder it will not work as that probably is not implemented yet so just have everything in the same folder. you would need to set up another one for the defs autoload as well but you can do this in the same file using -vdmap command and following this step. now save and your done.