add nydus image artifact

Artifact manifest is a nice way to offer the ability of enhancing
registry as well as applying minimum modifications to client tools.

Nydus image is an alternative representation of container image,
providing benefits like making starting containers as fast as
possible.

We'd like to have nydus image persist as a new artifact like helm
charts.

Signed-off-by: Peng Tao <<tao.peng@linux.alibaba.com>>
Signed-off-by: Liu Bo <bo.liu@linux.alibaba.com>

artifact-manifest/artifact-manifest.md

@@ -143,6 +143,10 @@ A CNAB may also be persisted with configuration information, along with a refere
 
 ![Wordpress CNAB](media/wordpress-cnab.svg)
 
+### Nydus Image
+
+A Nydus image is an alternative representation of container image, providing benefits like lazy-loading of images, etc.  Since it can be stored independently or along with the OCI image, their references are loose references.
+
 ## Supported Scenarios
 
 The main scenarios include:
@@ -381,6 +385,51 @@ As the `oci-reg copy` command is executed, the graph of references are expanded.
 }
 ```
 
+#### Nydus Image Reference
+
+```
+{
+  "schemaVersion": 2,
+  "mediaType": "application/vnd.oci.artifact.manifest.v1+json",
+  "artifactType": "application/vnd.cncf.nydus.v1",
+  "config": {
+    "mediaType": "application/vnd.oci.image.manifest.v1.config.json",
+    "digest": "sha256:9e988712154fcc2ceda5602eb1d98c1f28299ba6fbf0be49d3717c35a2d76674",
+    "size": 1102
+  },
+  "blobs": [
+    {
+      "mediaType": "application/vnd.cncf.nydus.bootstrap.v1.tar+gzip",
+      "digest":
+"sha256:f6bb0822fe567c98959bb87aa316a565eb1ae059c46fa8bba65b573b4489b44d",
+      "size": 32654
+    },
+    {
+      "mediaType": "application/vnd.cncf.nydus.blob.v1",
+      "digest":
+"sha256:f6bb0822fe567c98959bb87aa316a565eb1ae059c46fa8bba65b573b4489b44d",
+      "size": 72832
+    },
+    {
+      "mediaType": "application/vnd.cncf.nydus.blob.v1",
+      "digest":
+"sha256:f6bb0822fe567c98959bb87aa316a565eb1ae059c46fa8bba65b573b4489b44d",
+      "size": 928324
+    }
+  ],
+  "references": [
+    {
+      "artifact": "mysql:8",
+      "artifactType": "application/vnd.oci.image.manifest.v1.config.json",
+      "mediaType": "application/vnd.oci.image.manifest.v1.config.json",
+      "digest":
+"sha256:3c3a4604a545cdc127456d94e421cd355bca5b528f4a9c1905b15da2eb4a4c6b",
+      "size": 16724
+    }
+  ]
+}
+```
+
 ### Deletion
 
 Distribution-spec APIs will provide standard delete operations, including options for deleting referenced artifacts, or blocking a delete as the artifact is referenced by other artifacts. The `oci.artifact.manifest` collection will provide the information, as defined by the artifact author` for how an artifact should be handled for delete operations. The registry, nor the `oci-reg` cli would need to know about specific artifact implementations.
@@ -499,4 +548,4 @@ This specification defines the following annotation keys, intended for but not l
 
 ## Open Questions
 
-Should the references collection support additional types, like loose urls
+Should the references collection support additional types, like loose urls