Update MUTATIONS.md

doc/MUTATIONS.md

@@ -96,6 +96,7 @@ Note that **all new traits that can be obtained through mutation must be purifia
   "mixed_effect": false,                      // Whether the trait has both positive and negative effects.  This is purely declarative and is only used for the user interface (default: false).
   "description": "Nothing gets you down!",    // In-game description.
   "starting_trait": true,                     // Can be selected at character creation (default: false).
+  "random_at_chargen": false,                 // (Optional) Starting traits can be randomly assigned to NPCs during chargen.  This options prevents that (default: true).
   "valid": false,                             // Can be mutated ingame (default: true).  Note that prerequisites can even mutate invalid mutations.
   "purifiable": false,                        // Sets if the mutation be purified (default: true).
   "profession": true,                         // Trait is a starting profession special trait (default: false).