Add a TODO

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Lighting/Volumetrics/Resources/VolumetricLighting.compute

         // TODO: piecewise linear.
         float3 scattering = _GlobalFog_Scattering;
         float  extinction = _GlobalFog_Extinction;
+        
+        // TODO: define a function ComputeGlobalFogCoefficients(float3 centerWS),
+        // which allows procedural definition of extinction and scattering.
 
     #if ENABLE_REPROJECTION
         // This is a sequence of 7 equidistant numbers from 1/14 to 13/14.