Update documentation for spibb

[yzgyyang]

Hi! I used this library for my project and wrote this article. I implemented the spi bit banging functions myself without realizing there exists an abstraction in this library! I already mentioned this in my post, and am wondering if you could update documentations for spibb when you are free so that more people can see and use it?

[evadot]

Yes I have branch somewhere when I started some real documentation that I need to polish and publish somewhere.