Update snake_functions.py

Chapter06/snake_functions.py

 """
 Helping functions for snakes. 
+Note that snake here is 2-by-N array!
 """
 
 import numpy as np
 import scipy.interpolate
 
 def distribute_points(snake):
-    """ Distributes snake points equidistantly."""
+    """ Distributes snake points equidistantly. Expects snake to be 2-by-N array."""
     N = snake.shape[1]
     
     # Compute length of line segments.