Document which boards are supported

[oveddan]

I, for example, want to use this on the beaglebone. Is it supported there?

It would be great if at a minimum the readme said which platforms are supported.

[caternuson]

Unfortunately, this kind of information is nearly impossible to provide completely and also maintain current. The learn guide covers general usage requirements: https://learn.adafruit.com/adafruit-dotstar-leds

Closing since not a library code issue.