doc: improve addons.markdown copy

General improvements to the documentation in addons.markdown.

PR-URL: https://github.com/nodejs/node/pull/4320


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>