fix: add a doc about nydusify copy

Signed-off-by: BruceAko <[email protected]>

docs/nydusify.md

@@ -68,7 +68,7 @@ cat /path/to/backend-config.json
 }
 ```
 
-Note: the `endpoint` in the s3 `backend-config.json` **should not** contains the scheme prefix.
+Note: the `endpoint` in the s3 `backend-config.json` **should not** contain the scheme prefix.
 
 ``` shell
 nydusify convert \
@@ -178,7 +178,7 @@ nydusify check \
 
 The nydusify mount command can mount a nydus image stored in the backend as a filesystem. Now  the  supported backend types include Registry (default backend), s3 and oss. 
 
-When using Registy as the backend, you don't need specify the `--backend-type` .
+When using Registry as the backend, you don't need to specify the `--backend-type` .
 
 ``` shell
 nydusify mount \
@@ -204,6 +204,26 @@ nydusify copy \
 
 It supports copying OCI v1 or Nydus images, use the options `--all-platforms` / `--platform` to copy the images of specific platforms.
 
+## Export to / Import from local tarball
+
+All you need is to change the `source` or `target` parameter in `nydusify copy` command to a local file path, which must start with `file://`.
+
+``` shell
+# registry repository --> local tarball
+nydusify copy \
+  --source myregistry/repo:tag-nydus \
+  --target file:///home/user/repo-tag-nydus.tar
+```
+
+Absolute path is also supported.
+
+``` shell
+# local tarball --> registry repository
+nydusify copy \
+  --source file://./repo-tag-nydus.tar \
+  --target myregistry/repo:tag-nydus
+```
+
 ## Commit nydus image from container's changes
 
 The nydusify commit command can commit a nydus image from a nydus container, like `nerdctl commit` command.