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: #12312
Fixes: #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>

doc/api/fs.md

@@ -1442,6 +1442,9 @@ changes:
     pr-url: https://github.com/nodejs/node/pull/7897
     description: The `callback` parameter is no longer optional. Not passing
                  it will emit a deprecation warning.
+  - version: v6.0.0
+    pr-url: https://github.com/nodejs/node/pull/5616
+    description: The `options` parameter was added.
 -->
 
 * `path` {string|Buffer}