Write board implementation guide

[PMunch]

In order to make it easier for other people to implement their boards into the Ratel ecosystem the board implementation guide should be written.

[tsukimizake]

Hi. I'm trying to write a keyboard firmware for my crkbd with ratel.
I have to write pro micro support for ratel at first. I thought it can be relatively easy as its MCU is the same as teensy, but actually I'm at a loss. I'm currently copying teensy board support code to my project and rename teensy2 without changing the implementation, and stuck with Script Error: ~/.nimble/pkgs/ratel-0.2.1/private/helpers.nim:29:42 undeclared identifier: 'ratelBoard'
I surely want the doc.

[PMunch]

Hmm, I think this is actually caused by trying to copy more than anything else. But I will try to write a guide for this soon. Maybe as soon as this weekend or next week.