Add readme metadata specifying configs

[hombit]

Until we specify splits explicitly, datasets.load_dataset would mix "main" and "margin" catalog data, which would cause a small data duplication. We should specify README metadata, something like:

---
configs:
- config_name: default
  data_dir: "mmu_vipers_w4/dataset/"
- config_name: margin_10arcs
  data_dir: "mmu_vipers_w4_10arcs/dataset/"
---

https://huggingface.co/docs/hub/datasets-manual-configuration

[hombit]

I hope we can also support it from "hats"/"hats-import" side, but before it is implemented there, let's just compose README.md manually.
astronomy-commons/hats#615