Improve readme

[stefanpenner]

exiting stuff is complicated, we should likely teach how it works in node, and then explain how to use this library successfully in-light of the nodisms. Also documenting it, may lead someone to point out what we missed.

some confusions we likely need to be sure to cover:

• process.exit usage not consistent between onExit and on('exit' #12
• breaks other process.on('SIGINT', ...) handlers sindresorhus/ora#27

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We should also describe in full detail the various rules and lifecycle events and how/when they fire, and how this library works with them.