Add community files

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: >-
+  Squirrel: A Python library that enables ML teams to
+  share, load, and transform data in a collaborative,
+  flexible, and efficient way
+message: >-
+  If you use this software, please cite it using the
+  metadata from this file.
+type: software
+authors:
+  - given-names: Alireza Sohofi
+    affiliation: Merantix Momentum
+  - given-names: Tiansu Yu
+    affiliation: Merantix Momentum
+  - given-names: Alp Aribal
+    affiliation: Merantix Momentum
+  - given-names: Winfried Loetzsch
+    affiliation: Merantix Momentum
+  - given-names: Squirrel Developer Team
+  - given-names: Thomas Wollmann
+    affiliation: Merantix Momentum
+    orcid: 'https://orcid.org/0000-0002-4741-3844'

CODE-OF-CONDUCT.md

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+Check out [Code of Conduct](https://squirrel-core.readthedocs.io/en/latest/usage/code_of_conduct.html) in the official documentation.

CONTRIBUTING.md

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+Check out [contribution guide](https://squirrel-core.readthedocs.io/en/latest/usage/contribute.html) in the official documentation.