Adding a reference to Graphviz in the documentation.
Adding a reference to Graphviz in the documentation.

ViDA-NYU/reprozip,JosuaKrause/reprozip,JosuaKrause/reprozip,JosuaKrause/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,JosuaKrause/reprozip