added stringdoc for mex algorithm

fastsnake/algorithms/mex.py

@@ -1,4 +1,12 @@
 def mex(array):
+    """
+    The MEX (minimum excluded) of an array is the smallest non-negative integer that 
+    does not belong to the array. Example:
+
+    The MEX of [2,2,1] is 0, because 0 does not belong to the array.
+    The MEX of [3,1,0,1] is 2, because 0 and 1 belong to the array, but 2 does not.
+    The MEX of [0,3,1,2] is 4 because 0, 1, 2 and 3 belong to the array, but 4 does not.
+    """
     # Create a dictionary to store the frequency of each element
     freq_map = {}