Added JOSS citation link to README.md

Author: Martin D. Weinberg

CITATION.cff

@@ -0,0 +1,34 @@
+cff-version: "1.2.0"
+authors:
+- family-names: Petersen
+  given-names: Michael S.
+  orcid: "https://orcid.org/0000-0003-1517-3935"
+- family-names: Weinberg
+  given-names: Martin D.
+  orcid: "https://orcid.org/0000-0003-2660-2889"
+doi: 10.5281/zenodo.15164151
+message: If you use this software, please cite our article in the
+  Journal of Open Source Software.
+preferred-citation:
+  authors:
+  - family-names: Petersen
+    given-names: Michael S.
+    orcid: "https://orcid.org/0000-0003-1517-3935"
+  - family-names: Weinberg
+    given-names: Martin D.
+    orcid: "https://orcid.org/0000-0003-2660-2889"
+  date-published: 2025-05-05
+  doi: 10.21105/joss.07302
+  issn: 2475-9066
+  issue: 109
+  journal: Journal of Open Source Software
+  publisher:
+    name: Open Journals
+  start: 7302
+  title: "EXP: a Python/C++ package for basis function expansion methods
+    in galactic dynamics"
+  type: article
+  url: "https://joss.theoj.org/papers/10.21105/joss.07302"
+  volume: 10
+title: "EXP: a Python/C++ package for basis function expansion methods
+  in galactic dynamics"

README.md

@@ -96,3 +96,8 @@ This is only one example of many possible uses.  There are many
 variants to this work flow, of course, and I expect that you will
 invent some interesting ones.
 
+## Citation
+
+If you use EXP in your research, please cite the following paper:
+
+[![DOI](https://joss.theoj.org/papers/10.21105/joss.07302/status.svg)](https://doi.org/10.21105/joss.07302)