[skip ci] add: citation

Author: kozistr

CITATION.cff

@@ -0,0 +1,9 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - family-names: Kim
+    given-names: Hyeongchan
+    orcid: https://orcid.org/0000-0002-1729-0580
+title: "pytorch_optimizer: Bunch of optimizer implementations in PyTorch with clean-code, strict types"
+version: 1.0.0
+date-released: 2022-01-29