fix: Add citation information for repository

CITATION.cff

@@ -1,13 +1,30 @@
+cff-version: 1.2.0
+message: "Please cite the following works when using this software."
+type: software
 authors:
-  - family-names: Proffitt
-    given-names: Mason
-  - family-names: Watts
-    given-names: Gordon
-title: "FuncADL: Functional Analysis Description Language"
-version: 1.0.0
-doi: 10.1051/epjconf/202125103068
-url: https://arxiv.org/abs/2103.02432
-journal: "EPJ Web Conf."
-volume: "251"
-pages: "03068"
-year: 2021
+- family-names: "Watts"
+  given-names: "Gordon"
+  orcid: "https://orcid.org/0000-0002-0753-7308"
+  affiliation: "University of Washington"
+title: "func_adl"
+repository-code: "https://github.com/iris-hep/func_adl"
+url: "https://github.com/iris-hep/func_adl"
+license: "MIT"
+references:
+  - type: article
+    authors:
+    - family-names: "Proffitt"
+      given-names: "Mason"
+      orcid: "https://orcid.org/0000-0003-0323-8252"
+      affiliation: "University of Washington"
+    - family-names: "Watts"
+      given-names: "Gordon"
+      orcid: "https://orcid.org/0000-0002-0753-7308"
+      affiliation: "University of Washington"
+    title: "FuncADL: Functional Analysis Description Language"
+    doi: 10.1051/epjconf/202125103068
+    url: https://arxiv.org/abs/2103.02432
+    year: 2021
+    volume: 251
+    pages: 03068
+    journal: "EPJ Web Conf."

README.md

@@ -93,7 +93,7 @@ class dd_jet:
 class dd_event:
     def Jets(self, bank: str) -> Iterable[dd_jet]:
         ...
-    
+
     def EventNumber(self, bank='default') -> int
         ...
 
@@ -204,3 +204,28 @@ For examples, see the `test_type_based_replacement` file. The class-level decora
 ## Development
 
 After a new release has been built and passes the tests you can release it by creating a new release on `github`. An action that runs when a release is "created" will send it to `pypi`.
+
+## Citation
+
+The preferred BibTeX entry for citation of `func_adl` includes both the software repository and the EPJ Web Conf. paper:
+
+```bibtex
+@software{func_adl,
+    author = {Gordon Watts},
+    title = "{func\_adl}",
+    url = {https://github.com/iris-hep/func_adl}
+}
+
+@article{Proffitt:2021wfh,
+    author = "Proffitt, Mason and Watts, Gordon",
+    title = "{FuncADL: Functional Analysis Description Language}",
+    eprint = "2103.02432",
+    archivePrefix = "arXiv",
+    primaryClass = "physics.data-an",
+    doi = "10.1051/epjconf/202125103068",
+    journal = "EPJ Web Conf.",
+    volume = "251",
+    pages = "03068",
+    year = "2021"
+}
+```