doc: add `added:` information for v8

PR-URL: https://github.com/nodejs/node/pull/6684
Ref: https://github.com/nodejs/node/issues/6578


Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>