Adding citation

CITATION.cff

@@ -0,0 +1,32 @@
+cff-version: "1.2.0"
+authors:
+- family-names: Filot
+  given-names: I. A. W.
+  orcid: "https://orcid.org/0000-0003-1403-8379"
+contact:
+- family-names: Filot
+  given-names: I. A. W.
+  orcid: "https://orcid.org/0000-0003-1403-8379"
+doi: 10.5281/zenodo.8104498
+message: If you use this software, please cite our article in the
+  Journal of Open Source Software.
+preferred-citation:
+  authors:
+  - family-names: Filot
+    given-names: I. A. W.
+    orcid: "https://orcid.org/0000-0003-1403-8379"
+  date-published: 2023-07-06
+  doi: 10.21105/joss.05417
+  issn: 2475-9066
+  issue: 87
+  journal: Journal of Open Source Software
+  publisher:
+    name: Open Journals
+  start: 5417
+  title: "EDP: a program for projecting electron densities from VASP
+    onto planes"
+  type: article
+  url: "https://joss.theoj.org/papers/10.21105/joss.05417"
+  volume: 8
+title: "EDP: a program for projecting electron densities from VASP onto
+  planes"

README.md

@@ -12,6 +12,23 @@
 * Bugs and feature requests are ideally submitted via the [gitlab issue tracker](https://github.com/ifilot/edp/issues).
 * The development team can also be reached by email via i.a.w.filot@tue.nl.
 
+If you make use of this program, please use the following to cite it:
+
+```bibtex
+@article{Filot2023,
+    doi = {10.21105/joss.05417},
+    url = {https://doi.org/10.21105/joss.05417},
+    year = {2023},
+    publisher = {The Open Journal},
+    volume = {8},
+    number = {87},
+    pages = {5417},
+    author = {I.a.w. Filot},
+    title = {EDP: a program for projecting electron densities from VASP onto planes},
+    journal = {Journal of Open Source Software}
+}
+```
+
 ## Purpose
 
 EDP is a C++ program designed to project the electron density as stored in a