Improved readme

README.md

@@ -8,10 +8,13 @@ the last step, the Total Position Spread tensor is computed.
 
 It evaluates the matrix in open, and periodic boundary conditions.
 
+You can compare the matrix built with this program the results obtained
+[here](http://phyweb.phys.soton.ac.uk/teach/year4/notes/phys6017/Material/Lectures/lecture8.pdf).
+
 Citation
 ========
 
 All results published used this software should cite the following:
 
 - M. El Khatib, E. Fertitta, "Heisenberg: a program to the matrix calculation
-- of the Heisenberg model in spin chains" [![DOI](https://zenodo.org/badge/4178/muammar/heisenberg.png)](http://dx.doi.org/10.5281/zenodo.12904)
+  of the Heisenberg model in spin chains" [![DOI](https://zenodo.org/badge/4178/muammar/heisenberg.png)](http://dx.doi.org/10.5281/zenodo.12904)