Add GitHub CITATION.cff file

Co-authored-by: gitosaurus <[email protected]>

Author: Copilot

CITATION.cff

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+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+type: software
+title: nested-pandas
+authors:
+  - name: "LINCC Frameworks"
+    email: "[email protected]"
+repository-code: "https://github.com/lincc-frameworks/nested-pandas"
+url: "https://nested-pandas.readthedocs.io/"
+abstract: "An extension of pandas for efficient representation of nested associated datasets."
+keywords:
+  - pandas
+  - astronomy
+  - time-domain
+  - nested-data
+  - python
+license: MIT