Add LICENSE (CC-BY-IGO-4.0) and CITATION.cff

CITATION.cff

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+cff-version: 1.2.0
+message: If you use this work, please cite it as below.
+title: Notebooks
+type: dataset
+abstract: R Markdown notebooks
+url: https://github.com/iobis/notebooks
+license: CC-BY-IGO-4.0
+authors:
+  - given-names: Pieter
+    family-names: Provoost
+    orcid: "https://orcid.org/0000-0002-4236-0384"
+  - name: Intergovernmental Oceanographic Commission of UNESCO
+    alias: IOC-UNESCO
+    website: "https://obis.org"
+date-released: 2021-01-08

LICENSE

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+Creative Commons Attribution 4.0 International IGO License (CC-BY-IGO-4.0)
+
+Copyright (c) 2026 Intergovernmental Oceanographic Commission of UNESCO
+
+This work is licensed under the Creative Commons Attribution 4.0
+International IGO License.
+
+To view a copy of this license, visit:
+https://creativecommons.org/licenses/by/4.0/igo/
+
+You are free to:
+  - Share: copy and redistribute the material in any medium or format
+  - Adapt: remix, transform, and build upon the material for any purpose
+
+Under the following terms:
+  - Attribution: You must give appropriate credit, provide a link to the
+    license, and indicate if changes were made.
+
+No additional restrictions: You may not apply legal terms or technological
+measures that legally restrict others from doing anything the license permits.