Create CITATION.cff file for TensorFlow Federated.

PiperOrigin-RevId: 393790129

CITATION.cff

+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - affiliation: Google
+    given-names: "The TensorFlow Federated Authors"
+title: "TensorFlow Federated"
+version: 0.19.0
+date-released: 2018-12-12
+url: "https://github.com/tensorflow/federated"