update docs about save/load

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

README.md

@@ -131,6 +131,8 @@ Lots of commands and flags are currently missing. Pull requests are highly welco
 ## Features present in `nerdctl` but not present in Docker
 - Namespacing as in `kubectl --namespace=<NS>`: `nerdctl --namespace=<NS> ps`
 - [Lazy-pulling using Stargz Snapshotter](./docs/stargz.md): `nerdctl --snapshotter=stargz run`
+- Exporting Docker/OCI dual-format archives: `nerdctl save`
+- Importing OCI archives: `nerdctl load`
 
 Features ported from Podman:
 - Specifying a non-image rootfs: `nerdctl run -it --rootfs <ROOTFS> /bin/sh`

load.go

@@ -28,9 +28,10 @@ import (
 )
 
 var loadCommand = &cli.Command{
-	Name:   "load",
-	Usage:  "Load an image from a tar archive or STDIN",
-	Action: loadAction,
+	Name:        "load",
+	Usage:       "Load an image from a tar archive or STDIN",
+	Description: "Supports both Docker Image Spec v1.2 and OCI Image Spec v1.0.",
+	Action:      loadAction,
 	Flags: []cli.Flag{
 		&cli.StringFlag{
 			Name:    "input",

save.go

@@ -30,9 +30,10 @@ import (
 )
 
 var saveCommand = &cli.Command{
-	Name:   "save",
-	Usage:  "Save one or more images to a tar archive (streamed to STDOUT by default)",
-	Action: saveAction,
+	Name:        "save",
+	Usage:       "Save one or more images to a tar archive (streamed to STDOUT by default)",
+	Description: "The archive implements both Docker Image Spec v1.2 and OCI Image Spec v1.0.",
+	Action:      saveAction,
 	Flags: []cli.Flag{
 		&cli.StringFlag{
 			Name:    "output",