doc: updating REPLACEME tag during release

Add a paragraph to the releases.md guide to document replacing the
REPLACEME tag with the release version for new APIs.

PR-URL: https://github.com/nodejs/node/pull/7514


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>