Add citation files

CITATION.cff

@@ -0,0 +1,37 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - family-names: "McGibbon"
+    given-names: "Robert"
+  - family-names: "Helly"
+    given-names: "John"
+  - family-names: "Schaye"
+    given-names: "Joop"
+  - family-names: "Schaller"
+    given-names: "Matthieu"
+  - family-names: "Vandenbroucke"
+    given-names: "Bert"
+title: "SOAP: A Python Package for Calculating the Properties of Galaxies and Halos Formed in Cosmological Simulations"
+doi: "10.21105/joss.08252"
+date-released: "2025-07-01"
+preferred-citation:
+  type: article
+  authors:
+    - family-names: "McGibbon"
+      given-names: "Robert"
+    - family-names: "Helly"
+      given-names: "John"
+    - family-names: "Schaye"
+      given-names: "Joop"
+    - family-names: "Schaller"
+      given-names: "Matthieu"
+    - family-names: "Vandenbroucke"
+      given-names: "Bert"
+  doi: "10.21105/joss.08252"
+  journal: "The Journal of Open Source Software"
+  month: 7
+  year: 2025
+  title: "SOAP: A Python Package for Calculating the Properties of Galaxies and Halos Formed in Cosmological Simulations"
+  volume: 10
+  issue: 111
+  start: 8252

README.md

@@ -1,13 +1,15 @@
 # SOAP: Spherical Overdensity and Aperture Processor
 
-[![DOI](https://joss.theoj.org/papers/10.21105/joss.08252/status.svg)](https://doi.org/10.21105/joss.08252)
+[![DOI](https://joss.theoj.org/papers/10.21105/joss.08252/status.svg)](https://doi.org/10.21105/joss.08252) 
 
 This repository contains programs which can be used to compute
 properties of halos in spherical apertures in [SWIFT](https://swift.strw.leidenuniv.nl/) snapshots.
 The resulting output halo catalogues can be read using the
 [swiftsimio](https://swiftsimio.readthedocs.io/en/latest/)
 python package.
 
+Please cite SOAP using the [JOSS paper](https://ui.adsabs.harvard.edu/abs/2025JOSS...10.8252M)
+
 ## Installation
 
 The code is written in python and uses mpi4py for parallelism.