boards: arm: nrf52_pca20020: Add board documentation

A documentanion file is added for Thingy:52. This document only
discusses the currently supported features of the board. The omissions
include the audio features and sensors which don't have a working driver
in tree.

Signed-off-by: Aapo Vienamo <aapo.vienamo@iki.fi>
2 files changed