media-types.md: add foreign layer media type

Signed-off-by: Antonio Murdaca <[email protected]>

Author: runcom

media-types.md

@@ -6,6 +6,7 @@ The following `mediaType` MIME types are used by the formats described here, and
 - `application/vnd.oci.image.manifest.list.v1+json`: [Manifest list](manifest.md#manifest-list)
 - `application/vnd.oci.image.manifest.v1+json`: [Image manifest format](manifest.md#image-manifest)
 - `application/vnd.oci.image.serialization.rootfs.tar.gzip`: ["Layer", as a gzipped tar archive](serialization.md#creating-an-image-filesystem-changeset)
+- `application/vnd.oci.image.serialization.rootfs.foreign.tar.gzip`: ["Layer", as a gzipped tar that should never be pushed](serialization.md#creating-an-image-filesystem-changeset)
 - `application/vnd.oci.image.serialization.config.v1+json`: [Container config JSON](serialization.md#image-json-description)
 
 ## Compatibility Matrix