doc: fix v6 changelog

Not quite sure how, but quite a few of the commits were missing
from the original changelog generated for v6 relative to v5.11.0.
This updates the change log.

PR-URL: https://github.com/nodejs/node/pull/6435


Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>