adds example usage to spaces (#173)

gym/spaces/tuple_space.py

@@ -3,6 +3,9 @@ from gym import Space
 class Tuple(Space):
     """
     A tuple (i.e., product) of simpler spaces
+
+    Example usage:
+    self.observation_space = spaces.Tuple((spaces.Discrete(2), spaces.Discrete(3)))
     """
     def __init__(self, spaces):
         self.spaces = spaces