copying: add license and copyright information The REAME is updated to describe the copyright and license information. The COPYING file is for GNU GPL v2. The copyright information for usemem_migrate.c is taken from the header of migrate_pages.c of the numactl's test directory. The copyright infromation for usemem.c is taken from Andrew's agreement email replied to Fengguang. Signed-off-by: Aaron Lu <aaron.lu@intel.com> Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>