Add tournament_type to docstring

Author: meatballs

axelrod/tournament.py

@@ -22,6 +22,9 @@ def __init__(self, players, tournament_type=None, name='axelrod',
         ----------
         players : list
             A list of axelrod.Player objects
+        tournament_type : function
+            Returning a dictionary mapping a tuple of player index numbers to
+            an axelrod Match object
         name : string
             A name for the tournament
         game : axelrod.Game