Add changelog

Documentation/release/dev/improve-amr-selection-support.md

+## Improve vtkUniformGridAMR Selection Support
+
+vtkExtractSelection now properly works for vtkUniformGridAMR subclasses.
+This was achieved by Modifying vtkDataObjectTree to support vtkUniformGridAMR for CopyStructure, GetDataSet, SetDataSet.
+Also when creating the hierarchy of a vtkUniformGrid, now all composite indices are in a level are included.
+Finally, as we prepare to deprecate vtkMultiBlockDataSet, if the input is a vtkUniformGridAMR, vtkExtractSelection now
+generates a vtkPartitionedDataSetCollection instead of a vtkMultiBlockDataSet.