Add PubMed Central Open Access dataset
In the framework of BigScience:
- bigscience-workshop/data_tooling#121

I have created this dataset as a community dataset: https://huggingface.co/datasets/albertvillanova/pmc_open_access

However, I was wondering that it may be more appropriate to move it under an org namespace: `pubmed_central` or `pmc`
This way, we could add other datasets I'm also working on: Author Manuscript Dataset, Historical OCR Dataset, LitArch Open Access Subset.

What do you think? @lhoestq @mariosasko  Why not ! Having them under such namespaces would also help people searching for this kind of datasets.
We can also invite people from pubmed at one point DONE: https://huggingface.co/datasets/pmc/open_access
## Adding a Dataset
- **Name:** PubMed Central Open Access
- **Description:** The PMC Open Access Subset includes more than 3.4 million journal articles and preprints that are made available under license terms that allow reuse.
- **Paper:** *link to the dataset paper if available*
- **Data:** https://www.ncbi.nlm.nih.gov/pmc/tools/openftlist/
- **Motivation:** *what are some good reasons to have this dataset*

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