Add some documentation describing how to document tests,
with both the legacy way and via testplan.

Acked-by: Bhanuprakash Modem <bhanuprakash.modem@intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>