🔧 Add CITATION.cff file for academic citations (#681)

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: Typer
+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/typer'
+url: 'https://typer.tiangolo.com'
+abstract: >-
+  Typer, build great CLIs. Easy to code. Based on Python type hints.
+keywords:
+  - typer
+  - click
+license: MIT