[ADD] function Documentation

Author: Jeffresh

sigmoid_function.m

@@ -1,7 +1,10 @@
 function [sigm,d_sigm] = sigmoid_function(x)
-%UNTITLED Summary of this function goes here
-%   Detailed explanation goes here
+%SIGMOID_FUNCTION Function to calculate de sigmoid function and his derivate.
+
 sigm = 1./(1+exp(-x));
+
+% the derivate expression is expresed in this way to improve the 
+% eficiency. Look the way in which is used in the code and you will understand.
 d_sigm = x.*(1-x);
 end