doc: add missing changelog entry for fs.readdir()

Document that the `options` parameter of `fs.readdir()` was added
in Node.js 6.0.0.

PR-URL: https://github.com/nodejs/node/pull/12312
Fixes: https://github.com/nodejs/node/issues/12299


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>