doc: add topic - event loop, timers, nextTick()

Adds a new topic that provides an overview of the event loop, timers, and
`process.nextTick()` that is based upon a NodeSource "Need to Node" presentation
hosted by @trevnorris: Event Scheduling and the Node.js Event
Loop (https://nodesource.com/resources).

PR-URL: nodejs#4936
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Calvin W. Metcalf <calvin.metcalf@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>