add CITATION.cff and update README (#21)

CITATION.cff

@@ -0,0 +1,49 @@
+cff-version: "1.2.0"
+authors:
+- family-names: Rogowski
+  given-names: Marcin
+  orcid: "https://orcid.org/0000-0002-5662-2082"
+- family-names: Dalcin
+  given-names: Lisandro
+  orcid: "https://orcid.org/0000-0001-8086-0155"
+- family-names: Hammond
+  given-names: Jeff R.
+  orcid: "https://orcid.org/0000-0003-3181-8190"
+- family-names: Keyes
+  given-names: David E.
+  orcid: "https://orcid.org/0000-0002-4052-7224"
+contact:
+- family-names: Rogowski
+  given-names: Marcin
+  orcid: "https://orcid.org/0000-0002-5662-2082"
+doi: 10.5281/zenodo.8143862
+message: If you use this software, please cite our article in the
+  Journal of Open Source Software.
+preferred-citation:
+  authors:
+  - family-names: Rogowski
+    given-names: Marcin
+    orcid: "https://orcid.org/0000-0002-5662-2082"
+  - family-names: Dalcin
+    given-names: Lisandro
+    orcid: "https://orcid.org/0000-0001-8086-0155"
+  - family-names: Hammond
+    given-names: Jeff R.
+    orcid: "https://orcid.org/0000-0003-3181-8190"
+  - family-names: Keyes
+    given-names: David E.
+    orcid: "https://orcid.org/0000-0002-4052-7224"
+  date-published: 2023-07-19
+  doi: 10.21105/joss.05444
+  issn: 2475-9066
+  issue: 87
+  journal: Journal of Open Source Software
+  publisher:
+    name: Open Journals
+  start: 5444
+  title: "shmem4py: OpenSHMEM for Python"
+  type: article
+  url: "https://joss.theoj.org/papers/10.21105/joss.05444"
+  volume: 8
+title: "shmem4py: OpenSHMEM for Python"
+

README.rst

@@ -8,6 +8,8 @@ shmem4py: OpenSHMEM for Python
    :target: https://github.com/mpi4py/shmem4py/actions/workflows/containers.yml
 .. image::  https://readthedocs.org/projects/shmem4py/badge/?version=latest
    :target: https://shmem4py.readthedocs.io/en/latest/
+.. image:: https://joss.theoj.org/papers/10.21105/joss.05444/status.svg
+   :target: https://doi.org/10.21105/joss.05444
 
 Overview
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