[doc][handbook] Add a comment that there is the shallow water equation. Add a...

[doc][handbook] Add a comment that there is the shallow water equation. Add a few sentences on VTK format.

doc/handbook/5_structure.tex

@@ -87,7 +87,7 @@
   [.\node[FirstLevel] {freeflow};
     [.\node[SecondLevel] {\emph{models}};
       \node[ThirdLevel] {Single-phase free flow models using Navier-Stokes
-                         and eddy-viscosity based Reynolds-averaged Navier-Stokes turbulence models.};
+                         and eddy-viscosity based Reynolds-averaged Navier-Stokes turbulence models and shallow water equation model.};
     ]
   ]
   [.\node[FirstLevel] {geomechanics};

doc/handbook/6_inputoutput.tex

@@ -39,7 +39,8 @@ Furthermore, petrophysical properties can be defined (for each cell), by using e
 An example can be found in \texttt{dumux/test\allowbreak/porousmediumflow/2p/implicit/cornerpoint}.
 
 \subsubsection{VTK file format}
-
+VTK format uses ASCII and XML format. It is mostly used by \Dumux for output purposes and can be visualized by programs such as Paraview. Using VTK files to input grid and parameter data is also possible.
+An example can be found in \texttt{dumux/test\allowbreak/io/gridmanager}.
 
 \subsubsection{Other Grid Formats}
 Grid formats other than DGF, MSH, GRDECL, or VTK will have to be converted to the DGF, MSH, GRDECL, or VTK format before they can be used in \Dumux.