Add citation.cff (#464)

## Changes
Zenodoから引用するためのファイルを追加。

## 重要
ソースコードには変更なし

Author: K-Suzuki-Jij

CITATION.cff

@@ -0,0 +1,20 @@
+cff-version: 1.2.0
+message: "If you use this software, please cite it as below."
+authors:
+  - family-names: "Nishimura"
+    given-names: "Kohji"
+    affiliation: "Jij Inc."
+  - family-names: "Sakamoto"
+    given-names: "Yoshiki"
+    affiliation: "Jij Inc."
+  - family-names: "Shimizu"
+    given-names: "Taro"
+    affiliation: "Jij Inc."
+  - family-names: "Suzuki"
+    given-names: "Kohei"
+    affiliation: "Jij Inc."
+    orcid: "https://orcid.org/0009-0005-1025-8608"
+  - family-names: "Yamashiro"
+    given-names: "Yu"
+    affiliation: "Jij Inc."
+title: "OpenJij"