Add citation file

I think it follows the standard.

CITATION.cff

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+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+
+cff-version: 1.2.0
+title: Finite element Particle Kinetic Code
+message: >-
+  If you use this software, please cite it using the
+  metadata from this file.
+type: software
+authors:
+  - given-names: Jorge
+    family-names: Gonzalez
+    email: jorge.gonzalez@upm.es
+    affiliation: Universidad Politécnica de Madrid
+    orcid: 'https://orcid.org/0000-0001-7905-5001'
+repository-code: 'https://gitlab.com/JorgeGonz/fpakc'
+abstract: >-
+  Welcome to fpakc (Finite element PArticle Kinetic Code), a
+  modern object oriented Fortran open-source code for
+  particle simulations of plasma and gases. This code works
+  by simulating charged and neutral particles, following
+  their trajectories, collisions and boundary conditions
+  imposed by the user.
+
+  One of our aims is to make a code easy to maintain as well
+  as easy to use by a variety of reserchers and students.
+
+  This code is currenlty in very early steps of development.
+
+  The code aims to be easy to maintain and easy to use,
+  allowing its application from complex problems to easy
+  examples that can be used, for example, as teaching
+  exercises.
+
+  Parallelization techniques such as OpenMP, MPI will be
+  used to distribute the cpu load. We aim to make fpakc GPU
+  compatible in the future.
+
+  The codefpakc makes use of finite elements to generate
+  meshes in complex geometries. Particle properties are
+  deposited in the nodes and cells of the mesh. The
+  electromagnetic field, with the boundary conditions
+  imposed by the user, is solved also in this mesh.
+keywords:
+  - particle-in-cell
+  - plasma
+  - finite elements
+license: GPL-3.0
+version: beta
+date-released: '2025-10-01'