EDIT: THE FOLLOWING STEPS ARE NO LONGER RECOMMENDED. PLEASE SEE THE NEW STEPS A FEW POSTS DOWN FOR A MORE EFFICIENT WAY OF DOING THIS!
I'm certainly not a DECORATE guru but this method worked for me:
1. Create a new DECORATE entry in your WAD
2. Go to this page and copy the entire actor definition (For now, ignore the warnings at the top of the page)
3. Paste the actor definition into your new DECORATE entry
4. Add the following line above Player.StartItem "Pistol":Player.StartItem "Chainsaw"This overrides the old player start with the new one. Now, when you start the game, you'll have the fist, chainsaw, and pistol and the chainsaw will be the default starting/readied weapon. Basically the way it works is the first "Player.StartItem" entry determines which weapon will be the one held the moment you spawn.