doc: Edit app development instructions.

The instructions were made more to the point and clearer.
Optional and necessary steps are now easier to identify.

Change-Id: I3f4dc16b1dc0a7e64ae888ed050d266fa1d59645
Signed-off-by: L.S. Cook <leonax.cook@intel.com>
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>