Add doc/converted_Java_file_JP.md (#492)

* [Doc]: add doc/converted_Java_file_JP.md

* [Doc]: improve READMEs and change the title of converted_Java_file_JP.md

Author: yutaro-sakamoto

README.md

@@ -154,9 +154,10 @@ Run.
 java [PROGRAM-ID]
 ```
 
-## API reference of the runtime library
+## Documentation 
 
-The API reference of the runtime library `libcobj.jar` is available [here](https://opensourcecobol.github.io/opensourcecobol4j/javadoc/libcobj/index.html)
+* [The API reference of the runtime library `libcobj.jar`](https://opensourcecobol.github.io/opensourcecobol4j/javadoc/libcobj/index.html)
+* [opensource COBOL 4J: Java変換解説](./doc/converted_Java_file_JP.md)
 
 ## Development Progress
 

README_JP.md

@@ -156,9 +156,10 @@ cobj [COBOL source file]
 ```bash
 java [PROGRAM-ID]
 ```
-## ランタイムライブラリのAPIリファレンス
+## ドキュメント
 
-ランタイムライブラリである`libcobj.jar`のAPIリファレンスは[こちら](https://opensourcecobol.github.io/opensourcecobol4j/javadoc/libcobj/index.html)から確認できます。
+* [ランタイムライブラリ`libcobj.jar`のAPIリファレンス](https://opensourcecobol.github.io/opensourcecobol4j/javadoc/libcobj/index.html)
+* [opensource COBOL 4J: Java変換解説](./doc/converted_Java_file_JP.md)
 
 ## 実装状況