Add README.md

README.md

+# Dune-Vtk
+File reader and writer for the VTK Format
+
+## Summary
+Provides structured and unstructured file writers for the VTK XML File Formats 
+that can be opened in the popular ParaView visualization application. Additionally
+a file reader is provided to import VTK files into Dune grid and data objects.
+
+## Requirements
+For the management of the grids the `dune-grid` module is required. Additionally
+for the data evaluation, the `dune-functions` module is used. 
+
+### Optional modules
+For tests and examples `dune-spgrid` and `dune-polygongrid` are suggested, that 
+support structured grid data or special element types.
+
+## Usage
+The VTK writer works similar to the dune-grid `VTKWriter`. It needs to be bound 
+to a GridView and then data can be added to the points or cells in the grid.
+Points are not necessarily grid vertices, but any coordinates places inside the 
+grid cells, so the data must be provided as GridViewFunction to allow the local
+evaluation in arbitrary local coordinates.
+
+```c++
+Grid grid = ...;
+VtkWriter<typename Grid::LeafGridView> vtkWriter(grid.leafGridView());
+
+auto fct = makeAnalyticGridViewFunction([](auto const& x) {
+  return std::sin(x[0]);
+});
+
+vtkWriter.addPointData(fct, "u_points");
+vtkWriter.addCellData(fct, "u_cells");
+
+vtkWriter.write("filename.vtu", Vtk::ASCII);
+```
+
+See also the `src/` directory for more examples.
+
+## Comparison with Dune::VTKWriter
+In Dune-Grid there is a VTK writer available, that is a bit different from the
+proposed one. A comparions:
+
+| Property          | Dune-Grid | Dune-Vtk |
++-------------------+-----------+----------+
+| VTK version       | 0.1       | 1.0      |
+| UnstructuredGrid  | x         | x        |
+| PolyData          | x         | -        |
+| StructuredGrid    | -         | x        |
+| RectilinearGrid   | -         | x        |
+| ImageData         | -         | x        |
+| ASCII             | x         | x        |
+| BASE64            | x         | -        |
+| APPENDED_RAW      | x         | x        |
+| APPENDED_BASE64   | x         | -        |
+| BASE64_COMPRESSED | -         | -        |
+| APPEDED_COMPRESSED| -         | x        |
+| Parallel files    | x         | x        |