doc: add guide for Node.js Timers

Refs: https://github.com/nodejs/docs/issues/76
PR-URL: https://github.com/nodejs/node/pull/6825


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>