update citation

Jisk Attema · Jisk Attema · commit 513678895278 · 2020-07-14T14:12:07.000+02:00
diff --git a/CITATION.cff b/CITATION.cff
@@ -21,4 +21,5 @@ license: "Apache-2.0"
 message: "If you use this software, please cite it using these metadata."
 title: "psrdada-python"
 version: "0.1.0"
+date-released: 2020-07-14
 ...
