🔧 Add Citation config

Author: tiangolo

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: FastAPI CLI
+message: >-
+  If you use this software, please cite it using the
+  metadata from this file.
+type: software
+authors:
+  - given-names: Sebastián
+    family-names: Ramírez
+    email: [email protected]
+identifiers:
+repository-code: 'https://github.com/tiangolo/fastapi-cli'
+url: 'https://fastapi.tiangolo.com'
+abstract: >-
+  Run FastAPI applications from the command line.
+keywords:
+  - fastapi
+  - typer
+  - fastapi-cli
+license: MIT