Add more details on the summation in the mutual information formula

[Datseris]

After working with students on using mutual information, and referring them to our book, I realized that it was not clear to them what the summants n, m are in equation 7.1. The summation goes over bins, while the students were summing over the indices of x and y.

I think our text does not explain enough the MI. It defines it and proceeds immediatelly to discuss its properties, but I think we should spend a couple of sentences discussing what the summation means and how one does it in practice.