added citation file

CITATION.cff

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+# This CITATION.cff file was generated with cffinit.
+# Visit https://bit.ly/cffinit to generate yours today!
+
+cff-version: 1.2.0
+title: 'IPSUR: Introduction to Probability and Statistics Using R'
+message: >-
+  If you use this software, please cite it using the
+  metadata from this file.
+type: software
+authors:
+  - given-names: G. Jay
+    family-names: Kerns
+    email: gkerns@ysu.edu
+    affiliation: Youngstown State University
+    orcid: 'https://orcid.org/0000-0001-9354-2842'
+repository-code: 'https://github.com/gjkerns/IPSUR'
+url: 'https://ipsur.org'
+abstract: >-
+  An introductory probability and statistics textbook built
+  with the R programming language, alongside other
+  supplementary materials.  The book is released under the
+  GNU Free Documentation License.
+license: GPL-3.0
+commit: 7beaac1a530208db40de5e05f16660a6aa3b0a07
+version: 4.0.1206
+date-released: '2022-12-06'