doc: fs constants for Node < v6.3.0 in fs.md

Add changelog history in `fs.access` for the changes introduced to `constants` in the `fs` module prior to Node v6.3.0. PR-URL: #12690 Fixes: #8044 Reviewed-By: Colin Ihrig <cjihrig@gmail.com> Reviewed-By: James M Snell <jasnell@gmail.com> Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

Author: anshulguleria

doc/api/fs.md

@@ -426,6 +426,13 @@ changes:
  pr-url: https://github.com/nodejs/node/pull/10739
  description: The `path` parameter can be a WHATWG `URL` object using `file:`
  protocol. Support is currently still *experimental*.
+ - version: v6.3.0
+ pr-url: https://github.com/nodejs/node/pull/6534
+ description: The constants like `fs.R_OK`, etc which were present directly
+ on `fs` were moved into `fs.constants` as a soft deprecation.
+ Thus for Node `< v6.3.0` use `fs` to access those constants, or
+ do something like `(fs.constants || fs).R_OK` to work with all
+ versions.
 -->
 
 * `path` {string|Buffer|URL}