update readme

main.m

 %% Compilation
 
 clc
-mex PB2_mex.cpp -output PB2
-mex divergence_mex.cpp -output divergence
-mex gradient_mex.cpp -output gradient
+mex sources/PB2_mex.cpp -output PB2
+mex sources/divergence_mex.cpp -output divergence
+mex sources/gradient_mex.cpp -output gradient
 
 %% Script de test (equation 4.1)