Added description for 'docker run' command, -c/--cpu-shares flag

Signed-off-by: Shishir Mahajan <shishir.mahajan@redhat.com>
Docker run command with -c/--cpu-shares=0 will reflect container JSON 'CpuShares' as 1024, in sync with the file system value

Signed-off-by: Shishir Mahajan <shishir.mahajan@redhat.com>