Upload model card with the CLI
# 🚀 Feature request

<!-- A clear and concise description of the feature proposal.
     Please provide a link to the paper and code in case they exist. -->

Upload model card README.md with transformers CLI.

## Motivation

Some applications may require to upload multiple models. It would be nice to be able to upload associated model card with model/tokenizer/config files.

At the moment a PR is required making it more difficult for users to upload their models and nearly impossible for apps relying on training and uploading a lot of different models.

<!-- Please outline the motivation for the proposal. Is your feature request
     related to a problem? e.g., I'm always frustrated when [...]. If this is related
     to another GitHub issue, please link here too. -->

## Your contribution

<!-- Is there any way that you could help, e.g. by submitting a PR?
     Make sure to read the CONTRIBUTING.MD readme:
     https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md -->

I could help propose a PR for the client side.

This is not currently documented (cc @sgugger) but it's already possible, see for instance https://huggingface.co/Helsinki-NLP/opus-mt-fr-en

At some point it might become the preferred way of publishing a model card too.<|||||>Will update the tutorial today.<|||||>Thanks, this is great!