adds example usage to spaces (#173)

gym/spaces/box.py

@@ -7,6 +7,9 @@ class Box(gym.Space):
     """
     A box in R^n.
     I.e., each coordinate is bounded.
+
+    Example usage:
+    self.action_space = spaces.Box(low=-10, high=10, shape=(1,))
     """
     def __init__(self, low, high, shape=None):
         """

gym/spaces/discrete.py

@@ -6,6 +6,9 @@ from gym.spaces import prng
 class Discrete(gym.Space):
     """
     {0,1,...,n-1}
+
+    Example usage:
+    self.observation_space = spaces.Discrete(2)
     """
     def __init__(self, n):
         self.n = n

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