vtkHDFReader: add documentation

Documentation/release/dev/add-VTKHDF-transient-support.md

+## VTKHDF: Support for transient data
+
+The `VTKHDF` format now supports transient ImageData and UnstructuredGrid. Given the extent of the changes, the `VTKHDF` major version number has been incremented to 2.
+
+Schematically, the extra `Steps` group that contains metadata dictating how to read the transient data looks like this:
+
+![schema](transient_hdf_schema.png)
+
+Particularities:
+* Transient `vtkImageData` have an additional time dimension added to the arrays which should have the following shape [~nSteps, ZDim, YDim, XDim, NComponents] (where the `nSteps` dimension does not necessarily have to have as many entries as time steps and can use the offsetting mechanism to replay data without rewriting it)
+* Transient `vtkUnstructuredGrid` have flattened time-space arrays that are read in using the offsetting mechanism
+* The `Steps/Values` array holds the time values for each one of the steps
+* AMR type structures do not yet support transient data in the format
+
+Specific documentation related to the evolution of the format can  be found at [](https://kitware.github.io/vtk-examples/site/VTKFileFormats/).

IO/HDF/vtkHDFReader.h

@@ -47,6 +47,10 @@ class vtkUnstructuredGrid;
  * vtkDataSet types (image data and unstructured grid are currently
  * implemented) and serial as well as parallel processing.
  *
+ * Can also read transient data with directions and offsets present
+ * in a supplemental 'VTKHDF/Steps' group for vtkUnstructuredGrid and
+ * vtkImageData.
+ *
  */
 class VTKIOHDF_EXPORT vtkHDFReader : public vtkDataObjectAlgorithm
 {