Add CITATION.cff file with citation information

[euronion]

The version is hard coded. PyPSA somehow handles this automatically. Maybe easy to implement here as well @lkstrp ?

Checklist

• [n/a] Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in docs.
• [n/a] Unit tests for new features were added (if applicable).
• [n/a] A note for the release notes doc/release_notes.md of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license and have added my name to the doc/contributors.md.

[lkstrp]

I added it