manifest-list: non-manifest descriptors

For media that pertains to an image, like AppStream metadata, may need
to accompany a manifest-list, but is not a descriptor that points to a
manifest (i.e. and XML file).

This OPTIONAL `elements` field provides a place for these non-manifest
descriptors.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

manifest-list.md

@@ -21,10 +21,10 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 
 - **`manifests`** *array of objects*
 
-  This REQUIRED property contains a list of manifests for specific platforms.
-  While the property MUST be present, the size of the array MAY be zero.
+  This REQUIRED property contains a list of [manifests](manifest.md) for specific platforms.
+  While this property MUST be present, the size of the array MAY be zero.
 
-  Each object in `manifests` is a [descriptor](descriptor.md) with the following additional properties and restrictions:
+  Each object in `manifests` has the base properties of [descriptor](descriptor.md) with the following additional properties and restrictions:
 
   - **`mediaType`** *string*
 
@@ -65,6 +65,13 @@ For the media type(s) that this document is compatible with, see the [matrix][ma
 
         This OPTIONAL property specifies an array of strings, each specifying a mandatory CPU feature (for example `sse4` or `aes`).
 
+- **`elements`** *array of objects*
+
+  This OPTIONAL property contains a list of non-manifest objects.
+  The size of the array MAY be zero.
+
+  Each object in `elements` has the base properties of [descriptor](descriptor.md).
+
 - **`annotations`** *string-string map*
 
     This OPTIONAL property contains arbitrary metadata for the manifest list.