Document WS2812 module

@danb, I couldn't wait with merging this so I moved the docs into a separate issue. If you want to do this, that would be great but otherwise I can take this up as well.

The following discussion from !251 (merged) should be addressed:

• @rahix started a discussion: (+5 comments)

  This is great! I've still got some sewable neopixels lying around and I'd love to build a colorful wristband with them & this module :)

  Can you please rebase your changes on latest master and address the above comments so we can merge this?