Add ptq readme

tools/ptq/README.md

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+
+### Create PTQ Artefact / Run Calibration 
+Only needs to be done once per model
+```
+python -m tools.ptq.quantize \
+    --model_type flux_schnell 
+    --unet_path <path>/flux1-schnell.safetensors 
+    --clip_path clip_l.safetensors 
+    --t5_path t5xxl_fp16.safetensors  
+    --output flux_schnell_debug.json 
+    --calib_steps 16
+```
+
+### Create Quantized Checkpoint
+Uses the artefact from before + checkpoint to generate a quantized checkpoint based of a yml configuration. 
+This file defines which layers to quantize and what dtype they should use using regex. See `tools/ptq/configs/` for examples.
+```yaml
+# config.yml
+disable_list: ["*img_in*", "*final_layer*", "*norm*"]  # Keep these in BF16
+per_layer_dtype: {"*": "float8_e4m3fn"}                # Everything else to FP8
+```
+
+```
+python -m tools.ptq.checkpoint_merger 
+    --artefact flux_dev_debug.json 
+    --checkpoint <path>/flux1-dev.safetensors 
+    --config tools/ptq/configs/flux_nvfp4.yml 
+    --output <out_path>/flux1-nvfp4.safetensors
+    --debug
+```
+