Support Java 16 (#671)

[skip ci]

Author: nakamura-to

README.md

@@ -206,5 +206,6 @@ Major versions
 | Java 13 |        |   v    |
 | Java 14 |        |   v    |
 | Java 15 |        |   v    |
+| Java 16 |        |   v    |
 
   [apt]: https://www.jcp.org/en/jsr/detail?id=269