Add citation info

Author: inducer

CITATION.cff

@@ -0,0 +1,11 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+- family-names: "Kloeckner"
+  given-names: "Andreas"
+  orcid: "https://orcid.org/0000-0003-1228-519X"
+title: "islpy"
+version: 2022.1.2
+doi: 10.5281/zenodo.6345184
+date-released: 2022-03-10
+url: "https://github.com/inducer/islpy"

README.rst

@@ -10,6 +10,8 @@ islpy: Polyhedral Analysis from Python
 .. image:: https://badge.fury.io/py/islpy.png
     :alt: Python Package Index Release Page
     :target: https://pypi.org/project/islpy/
+.. image:: https://zenodo.org/badge/2021524.svg
+   :target: https://zenodo.org/badge/latestdoi/2021524
 
 islpy is a Python wrapper around Sven Verdoolaege's `isl
 <https://libisl.sourceforge.io/>`_, a library for manipulating sets and