Add changelog for CreateResult changes

docs/changelog/simplify-create-result.md

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+# Simplify creating results for Filters
+
+As part of the process of making VTK-m filters easier to write for newcomers
+whe have a couple of changes to make constructing the output `vtkm::cont::DataSet`
+easier.
+
+First we have moved the `CreateResult` functions out of the internals namespace
+and directly into `vtkm::filter`. This makes it clearer to developers that this
+was the 'proper' way to construct the output DataSet.
+
+Second we have streamlined the collection of `vtkm::filter::CreateResult` methods to
+require the user to provide less information and provide clearer names explaing what
+they do.
+
+To construct output identical to the input but with a new field you now just pass
+the `vtkm::filter::FieldMetadata` as a paramter instead of explictly stating
+the field association, and the possible cell set name:
+```cpp
+return CreateResult(input, newField, name, fieldMetadata);
+```
+
+To construct output identical to the input but with a cell field added you
+can now pass the `vtkm::cont::CellSet` as a paramter instead of explictly stating
+the field association, and the cell set name:
+```cpp
+return CreateResultFieldCell(input, newCellField, name, cellset);
+```