Add common voice
I have started working on adding this dataset. Hey @BirgerMoell - awesome that you started working on Common Voice. Common Voice is a bit special since, there is no direct download link to download the data. In these cases we usually consider two options:

1) Find a hacky solution to extract the download link somehow from the XLM tree of the website 
2) If this doesn't work we force the user to download the data himself and add a `"data_dir"` as an input parameter. E.g. you can take a look at how it is done for [this](https://github.com/huggingface/datasets/blob/66f2a7eece98d2778bd22bb5034cb7c2376032d4/datasets/arxiv_dataset/arxiv_dataset.py#L66) 

Also the documentation here: https://huggingface.co/docs/datasets/add_dataset.html?highlight=data_dir#downloading-data-files-and-organizing-splits (especially the "note") might be helpful. Let me know if you have any other questions I added a Work in Progress pull request (hope that is ok). I've made a card for the dataset and filled out the common_voice.py file with information about the datset (not completely).

I didn't manage to get the tagging tool working locally on my machine but will look into that later.

Left to do.

- Tag the dataset
- Add missing information and update common_voice.py

https://github.com/huggingface/datasets/pull/1886 Awesome! I left a longer comment on the PR :-) I saw that this current datasets package holds common voice version 6.1, how to add the new version 7.0 that is already available? Will me merged next week - we're working on it :-)
## Adding a Dataset
- **Name:** *common voice*
- **Description:** *Mozilla Common Voice Dataset*
- **Paper:** Homepage: https://voice.mozilla.org/en/datasets
- **Data:** https://voice.mozilla.org/en/datasets
- **Motivation:** Important speech dataset
- **TFDatasets Implementation**: https://www.tensorflow.org/datasets/catalog/common_voice
If interested in tackling this issue, feel free to tag @patrickvonplaten

Instructions to add a new dataset can be found [here](https://github.com/huggingface/datasets/blob/master/ADD_NEW_DATASET.md).
