Add General documentation style guidelines.

The style guidelines are mandatory patterns of writing designed to ease
the creation of documentation for the Zephyr project. All authors, which
includes the developers, must adhere to the style guide.

More detailed information, including examples, will be added shortly.

Change-Id: I198e5fe90b279560a60c028260b1fc8c972d322a
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>