Add release notes for vtkCompositePolyDataMapper

Documentation/release/dev/enhance-performance-of-abstract-composite-mapper.md

+## Enhance performance of vtkCompositePolyDataMapper
+
+The `vtkCompositePolyDataMapper` in VTK::RenderingCore module was improved to efficiently
+render large composite datasets. It performs as fast as `vtkCompositePolyDataMapper2`
+in VTK::RenderingOpenGL2 module. You no longer need to use `vtkCompositePolyDataMapper2` for its
+performance benefits.
+
+This refactor significantly impacted few VTK modules:
+- `vtkCompositePolyDataMapper` now has an API similar to `vtkCompositePolyDataMapper2`. You will most likely
+ end up renaming all occurences of `vtkCompositePolyDataMapper2` to `vtkCompositePolyDataMapper` in your C++/Python code.
+- `vtkCompositeSurfaceLICMapper` derives `vtkCompositePolyDataMapper` instead of `vtkCompositePolyDataMapper2`.
+- The OSPRay module uses `vtkCompositePolyDataMapper` instead of `vtkCompositePolyDataMapper2`.
+- `vtkVtkJSSceneGraphSerializer` uses `vtkCompositePolyDataMapper` instead of `vtkCompositePolyDataMapper2`.
+
+In order to minimize code duplication and maintenance burden, `vtkCompositePolyDataMapper2` and
+`vtkvtkOSPRayCompositePolyDataMapper2Node` are slated for removal in a future release and no longer
+used in VTK.