Lack of requirements for builder.py

**Describe the bug**
It is very unclear how to setup a python environment for builder.py as installing onnxruntime-genai-directml is not enough. This used to be included here: https://onnxruntime.ai/docs/genai/howto/build-model.html, but seems this was removed after moving instructions to the README.

**To Reproduce**
- hf auth login --token
- pip install onnxruntime-genai-directml
- python builder.py -m deepseek-ai/DeepSeek-R1-Distill-Llama-8B -p int4 -e dml -o onnx_model_builder\DeepSeek-R1-Distill-Llama-8B_int4_DML_ortgenai -c onnx_model_builder\ortgenai_model_cache --extra_options int4_algo_config=rtn
- missing pypi package errors.

**Expected behavior**
Run without chasing requirements needed for builder.py.

**Desktop:**
- OS: Windows 11
- Browser: Firefox
- Version: 0.9.1

Start with builder.py and the README, then reproduce the reported Windows command in a clean Python environment after installing onnxruntime-genai-directml. Compare the missing-package errors with the setup instructions and document the required requirements so the command runs without manual package hunting.

Written by the indexing model from the issue text.