Templates and explanation for adding a new model and example script
This PR adds:
- templates and explantations for all the steps needed to add a new model
- a simple template for adding a new example script (basically the current `run_squad` example).
- links to them in the `README` and `CONTRIBUTING` docs.

@LysandreJik and @rlouf, feel free to update if you want to add stuff or fix the wording.
Thanks @stefan-it, feel free to give your opinion on the explanation/templates as well, always happy to have your feedback