Add citation file. (#140)

Add citation file.

---------

Signed-off-by: Alexander Terenin <aterenin@users.noreply.github.com>
Co-authored-by: Viacheslav Borovitskiy <viacheslav.borovitskiy@gmail.com>

CITATION.bib

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+@article{mostowsky2024,
+      title = {The GeometricKernels Package: Heat and Matérn Kernels for Geometric Learning on Manifolds, Meshes, and Graphs},
+      author = {Peter Mostowsky and Vincent Dutordoir and Iskander Azangulov and Noémie Jaquier and Michael John Hutchinson and Aditya Ravuri and Leonel Rozo and Alexander Terenin and Viacheslav Borovitskiy},
+      year = {2024},
+      journal = {arXiv:2407.08086},
+}

CITATION.cff

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+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+title: "GeometricKernels"
+authors:
+- name: "GeometricKernels Contributors"
+preferred-citation:
+  type: "article"
+  title: "The GeometricKernels Package: Heat and Matérn Kernels for Geometric Learning on Manifolds, Meshes, and Graphs"
+  authors:
+  - family-names: "Mostowsky"
+    given-names: "Peter"
+  - family-names: "Dutordoir"
+    given-names: "Vincent"
+  - family-names: "Azangulov"
+    given-names: "Iskander"
+  - family-names: "Jaquier"
+    given-names: "Noémie"
+  - family-names: "Hutchinson"
+    given-names: "Michael John"
+  - family-names: "Ravuri"
+    given-names: "Aditya"
+  - family-names: "Rozo"
+    given-names: "Leonel"
+  - family-names: "Terenin"
+    given-names: "Alexander"
+  - family-names: "Borovitskiy"
+    given-names: "Viacheslav"
+  year: "2024"
+  journal: "arXiv:2407.08086"