Build 67: added brief notes for modders

Added a file I wrote last year but forgot to add to the ZIP, modders.txt, which describes how to add Mr Friendly-compatible name and description strings for any custom or overridden objects you include with a mod. If you do this for a mod, let me know and I can start a list of specially-supported mods!

To be clear Mr Friendly will already work fine with most existing Doom mods, this just lets you add your own amusing notes for objects unique to your mod.