Add `tldr_news` dataset
Hey @lhoestq, 
Sorry for opening a PR, I was following the guide [here](https://github.com/huggingface/datasets/blob/master/ADD_NEW_DATASET.md)! Thanks for the review anyway, I will follow the instructions you sent 😃  Thanks, we will update the guide ;)
This PR aims at adding support for a news dataset: `tldr news`.

This dataset is based on the daily [tldr tech newsletter](https://tldr.tech/newsletter) and contains a `headline` as well as a `content` for every piece of news contained in a newsletter.