selftests/resctrl: Update README about using kselftest framework to build/run resctrl_tests

resctrl_tests can be built or run using kselftests framework.
Add description on how to do so in README.

Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Shaopeng Tan <tan.shaopeng@jp.fujitsu.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>