fix: Add citation information for repository

* Add citation information for the code and keep the paper as a supporting
  reference.

Author: matthewfeickert

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."