Added release note for rectilinear grid cell data support

Documentation/release/dev/rectgrid-celldata.md

+## Rectilinear Grid Volume Rendering with Cell Data
+
+Fixed an issue in the volume mapper for rectilinear grids with cell scalars where the bounds of the
+dataset were incorrectly mapped to the texture space.
+
+|Before|After|
+|:--:|:--:|
+|![](./rectgrid_celldata_pre.png)|![](./rectgrid_celldata_fixed.png)|