Document LD_LIBRARY_PATH light filter issue

When rendering on linux, if the env variable LD_LIBRARY_PATH is not set or doesn't point to: `$HFS/dsolib` (where $HFS is the install directory of houdini) light shaders (decay, gobos, etc.) are not properly evaluated.

I don't consider this a bug, but I would encourage that this is documented somewhere prominent, as we had to debug this trial by error for quite some time.

LD_LIBRARY_PATH not set

LD_LIBRARY_PATH set to $HFS/dsolib

**Used Software Versions**
- Arnold:7.4.4.0
- Htoa 6.4.5.1
- Houdini 20.5.684

Review the existing Linux and Houdini setup or rendering documentation to find the most prominent place for this requirement. Document the LD_LIBRARY_PATH value needed for light shaders, with the Arnold, HtoA, and Houdini versions as context; done means users can find and follow the prerequisite without trial and error.

Written by the indexing model from the issue text.