DOC - Add node version warning.

[ulivz]

I have found that many users will encounter some unexpected errors caused by the wrong version of the Node.js, see:

• SyntaxError: Unexpected token function #21
• TypeError: promisify is not a function #39
• Async functions are not supported by Node <7.6 #51
• 我觉得应该添加一下Node版本信息？ #63

Although we have set minimum version at engines field of package.json, but NPM still existed some issues for that, See: npm/npm#12486

Of course, we can also check the node version at runtime, but it would be better to point out in the document that the user can avoid these problems in advance.