Implementations should support zstd

Signed-off-by: Brandon Mitchell <[email protected]>

manifest.md

@@ -85,10 +85,15 @@ Unlike the [image index](image-index.md), which contains information about a set
     - [`application/vnd.oci.image.layer.nondistributable.v1.tar+gzip`](layer.md#gzip-media-types)
 
     Manifests concerned with portability SHOULD use one of the above media types.
-    Implementations storing or copying image manifests MUST NOT error on encountering a `mediaType` that is unknown to the implementation.
-
     Entries in this field will frequently use the `+gzip` types.
 
+    Implementations SHOULD also support the following media types:
+
+    - [`application/vnd.oci.image.layer.v1.tar+zstd`](layer.md#zstd-media-types)
+    - [`application/vnd.oci.image.layer.nondistributable.v1.tar+zstd`](layer.md#zstd-media-types)
+
+    Implementations storing or copying image manifests MUST NOT error on encountering a `mediaType` that is unknown to the implementation.
+
     If the manifest uses a different media type than the above, it MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2], and MAY be registered with [IANA][iana].
 
   See [Guidelines for Artifact Usage](#guidelines-for-artifact-usage) for other uses of the `layers`.