[ADD] Documentation

Author: Jeffresh

cost_function.m

@@ -1,6 +1,5 @@
 function [c,d_c] = cost_function(so,sd)
-%COST_FUNCTION Summary of this function goes here
-%   Detailed explanation goes 
+%COST_FUNCTION Mean scuared error and his derivated.
 
 c =mean((so-sd).^2);
 d_c = so-sd;