Add new user to USERS.cff

[Florian Kotthoff]

Thanks for supporting the project and helping to improve its recognition by joining USERS.cff.
It helps the project quite a bit!

We will add you to the list of valued users.

Please, insert your information between the double quotes below - fill out at minimum "affiliation" 💜

family-names: "Kotthoff"
given-names: "Florian"
alias: "@FlorianK13"
affiliation: "OFFIS eV"
orcid: "https://orcid.org/0000-0003-3666-6122"

Thank you!