Add Zenodo badge, citation info

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: "pytools"
+version: 2022.1.7
+doi: 10.5281/zenodo.6533949
+date-released: 2022-05-04
+url: "https://github.com/inducer/pytools"

README.rst

@@ -10,6 +10,9 @@ Pytools: Lots of Little Utilities
 .. image:: https://badge.fury.io/py/pytools.png
     :alt: Python Package Index Release Page
     :target: https://pypi.org/project/pytools/
+.. image:: https://zenodo.org/badge/1575270.svg
+    :alt: Zenodo DOI for latest release
+    :target: https://zenodo.org/badge/latestdoi/1575270
 
 Pytools is a big bag of things that are "missing" from the Python standard
 library. This is mainly a dependency of my other software packages, and is