docs: update libcontainer README for external cgroups repo

[eyasy1217]

refs. #4638

[kolyshkin]

While at it, maybe you can add the missing "includes" section, which should clarify things.

[eyasy1217]

While at it, maybe you can add the missing "includes" section, which should clarify things.

Does that mean I should put the import statements inside the Container creation code?

+import (
+	"github.com/opencontainers/cgroups"
+	_ "github.com/opencontainers/cgroups/devices"
+	"github.com/opencontainers/cgroups/devices/config"
+	"github.com/opencontainers/runc/libcontainer/configs"
+	_ "github.com/opencontainers/runc/libcontainer/nsenter"
+	"github.com/opencontainers/runc/libcontainer/specconv"
+	"golang.org/x/sys/unix"
+	// ...
+)

+// ...

defaultMountFlags := unix.MS_NOEXEC | unix.MS_NOSUID | unix.MS_NODEV
var devices []*config.Rule
for _, device := range specconv.AllowedDevices {
	devices = append(devices, &device.Rule)
}

[kolyshkin]

Yes, I meant import, not includes :)

Ideally though, these code samples should be placed out of README and into some Go files so that they at least can be test-compiled. Or, README can point out to real code in runc which can also serve as an example.