Add utilities for processing ies light files

pxr/imaging/plugin/hdEmbree/pxrIES/README.md

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+# IES utilities
+
+Utilities for reading and using .ies files (IESNA LM-63 Format), which are used
+to describe lights.
+
+The files `ies.h` and `ies.cpp` are originally from
+[Cycles](https://docs.blender.org/manual/en/latest/render/cycles/index.html),
+which is part of the larger
+[Blender](https://projects.blender.org/blender/blender/) project, though with
+a the less restrictive Apache 2.0 license - see:
+
+- https://www.blender.org/about/license/
+
+## Version
+
+v4.1.1 ( 53c49589f311bd3c9e3ea8f62b3fa8fe8e5d2c8c )
+
+## Setup
+
+When updating IES, the following steps should be followed:
+
+1. Copy `intern/cycles/util/ies.h` and `intern/cycles/util/ies.cpp` over the
+   copies in `pxr/imaging/plugin/hdEmbree/pxrIES`.
+2. Apply `pxr-IES.patch` to update the source files with modifications for USD,
+   ie, from the USD repo root folder:
+
+   ```sh
+   patch -p1 -i pxr/imaging/plugin/hdEmbree/pxrIES/pxr-IES.patch
+   ```
+3. Commit your changes, noting the exact version of blender that the new ies
+   files were copied from.