Addresses #1.
- Credit and cite the original EchoXFlow dataset ([paper](https://arxiv.org/abs/2605.05447), [HF](https://huggingface.co/datasets/Ahus-AIM/EchoXFlow), [code](https://github.com/Ahus-AIM/EchoXFlow)).
- Set license to CC BY-NC-SA 4.0 and add the original `LICENSE` file.
- Document that this is a filtered 2-D B-mode subset and what is missing compared to the original.
