doc: update releases.md with new changelog structure

PR-URL: https://github.com/nodejs/node/pull/6503


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Robert Lindstaedt <robert.lindstaedt@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>