doc: add `added:` information for zlib

Ref: https://github.com/nodejs/node/issues/6578
PR-URL: https://github.com/nodejs/node/pull/6840


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Robert Jefe Lindstaedt <robert.lindstaedt@gmail.com>