add readme_path to config for HF model cards

[jettjaniak]

a stub of what we could use

---
language:
- en
license: apache-2.0
datasets:
- delphi-suite/stories
library_name: delphi
---

This model was trained using [Delphi](https://github.com/delphi-suite/delphi). See `training_config.json` and `run_context.json` for details.

[SrGonao]

Are you also adding to this PR the actual way to produce the model cards or will that be a follow up? otherwise looks good

[jettjaniak]

TODOs

• upload readme only to the main branch
• provide a default