Collect the rules of API design policy

[mrkn]

We import Gluon API mainly from Python's implementation.
Make our Gluon API appropriate to Ruby's programming style, we need to collect the rules of API design.

Collected rules

• Do not import function-style constructor from Python

[toddsundsted]

@mrkn what versions of Ruby do you want to target? are there any language changes in latter versions of Ruby that you want to avoid using?

[mrkn]

@toddsundsted Now I want to support 2.4 or higher because 2.3 is going to be EOL soon.
Moreover, I think it is enough we care the living versions of rubies only until we can release the stable version of mxnet.rb.