WIP: Add the guide level document about null vs undefined

[tetsuharuohzeki]

This attempt to fix #732.

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As a personal duty of maintaining a library like providing a toolkit utilities to handle nullabltily for JavaScript, I challenge to answer a long standing question - which should we use null - by this document.

I was asked many times from my colleagues that what should we use null or undefined when we design an interface. In almost case, my answer is that "null would be nice if you'd like to express to return a none value. It has a strong intent" with some (often a bunch of) reasons. I belive my answer would be a right decision but that does not stands by any investigations for that history and current situations of the ecosystem. So this guide presents some helps to you when you're worry about which you should use.