doc: updating REPLACEME tag during release

Checklist

• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc

Description of change

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

Ref: https://github.com/nodejs/node/pull/6864#issuecomment-220301714, #6578 (closed)

@addaleax @claudiorodriguez @evanlucas