pcm-server: add documentation
pcm-server: add documentation

This patch adds some basic documentation on the sensors and format of the output of pcm-server

Signed-off-by: Joseph Gasparakis <388c6eecced3fde82f39ad40e40c01863ad74ac6@intel.com>

opcm/pcm,opcm/pcm,opcm/pcm,opcm/pcm