Add SPI documentation shields.io badges

[hassila]

Would be neat with a SPI shields.io badge for the documentation that links to the relevant documentation page at SPI, WDYT?

[daveverwer]

Yep, we agree! It's being discussed here: #2119

I think the consensus is that what you have proposed here, a simple badge with a link to the documentation is exactly what we have settled on as how to implement this. I'll lock this one, though, to consolidate the threads.