Revise XUASTC LDR section in README Updated the description of XUASTC LDR and added details about distortion measurement.

diff --git a/README.md b/README.md
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@@ -117,7 +117,7 @@
 
 XUASTC LDR supports the following ASTC configurations: L/LA/RGB/RGBA CEMs; base+scale or RGB/RGBA direct; base+offset CEMs; Blue Contraction encoding; 1–3 subsets; all partition patterns; and single- or dual-plane modes. Here is the [XUASTC LDR specification](https://github.com/BinomialLLC/basis_universal/wiki/XUASTC-LDR-Specification-v1.0). Also see the [ASTC and XUASTC LDR Usage Guide](https://github.com/BinomialLLC/basis_universal/wiki/ASTC-and-XUASTC-LDR-Usage-Guide).
 
-XUASTC LDR 4x4 arithmetic bit rate vs. distortion graph - 151 test textures/images (the same test corpus we used to create [bc7e.ispc](https://github.com/richgel999/bc7enc_rdo)):
+XUASTC LDR 4x4 arithmetic bit rate vs. distortion graph - 151 test textures/images (the same test corpus we used to create [bc7e.ispc](https://github.com/richgel999/bc7enc_rdo)). Distortion was measured using [PSNR-HVS-M](https://pypi.org/project/psnr-hvsm/).
 <img width="1284" height="759" alt="image" src="https://github.com/user-attachments/assets/14dd6cee-4d0a-47a6-9cca-03c5e8e149ce" />
 
 Notes: