metrics: update documentation
metrics: update documentation

This commit adds a README file with general information
about metrics tests and how to execute them.

Fixes: #538

Signed-off-by: Mario Alfredo Carrillo Arevalo <962470f5d16f97233f38b7e7baf23f990a7fbec3@intel.com>

clearcontainers/tests,devimc/tests,MarioCarrilloA/tests,MarioCarrilloA/tests,devimc/tests,clearcontainers/tests,chavafg/tests,chavafg/tests