Adds link to model card metadata spec (#1171)
* Adds link to model card metadata spec
* Update src/huggingface_hub/templates/modelcard_template.md
Co-authored-by: Nathan Raw <nxr9266@g.rit.edu>
Co-authored-by: Lucain <lucainp@gmail.com>
Co-authored-by: Nathan Raw <nxr9266@g.rit.edu>