add citation information

Dieses Issue hat noch niemand übernommen.

I'd like to cite jupyter-matplotlib in something I'm working on. What is the preferred citation information for this repository? If I use Zotero to scrape the metadata this is what I get:

```
@misc{jupyter-matplotlib_2019,
title = {jupyter-matplotlib},
copyright = {BSD-3-Clause},
url = {https://github.com/matplotlib/jupyter-matplotlib},
abstract = {Matplotlib Jupyter Extension. Contribute to matplotlib/jupyter-matplotlib development by creating an account on GitHub.},
urldate = {2019-11-19},
publisher = {{Matplotlib Developers}},
month = nov,
year = {2019},
note = {original-date: 2016-07-21T14:38:56Z},
}
```

however, I'm not sure of how authorship should be determined? I could list all 13 contributors of this repository, or it could be "Matplotlib Developers", or something else? What would be best here?

I'm happy to contribute this information back into the repo in the readme or a citation file if you'd like too.

Beginne damit, die README des Repositorys und alle vorhandenen Zitationsmetadaten zu prüfen. Ermittle anhand der Repository-Informationen das bevorzugte Format für Autorenschaft und Zitierung und dokumentiere es anschließend in der README oder einer Zitationsdatei, damit Benutzer ipympl konsistent zitieren können.

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