Adding a CITATION file.

[Nikoleta-v3]

Hello and thank you for your package!

I am a PhD student and I am currently using your library for one of my research projects. I was wondering if you would be interested in creating a citation file so the library can be cited. Here is a very good blog post on why/who citing your software is good practice: https://www.software.ac.uk/blog/2016-10-06-encouraging-citation-software-introducing-citation-files

Let me know if you would like me to open a pull request 👍