descriptor: add urls property

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

Author: runcom

descriptor.md

@@ -32,13 +32,13 @@ The following fields contain the primary properties that constitute a Descriptor
   This property exists so that a client will have an expected size for the content before processing.
   If the length of the retrieved content does not match the specified length, the content SHOULD NOT be trusted.
 
-### Reserved
+- **`urls`** *array*
 
-The following field keys MUST NOT be used in descriptors specified in other OCI specifications:
+  This OPTIONAL property specifies a list of URLs from which this object MAY be downloaded.
 
-- **`urls`** *array*
+### Reserved
 
-  This key is RESERVED for future versions of the specification.
+The following field keys MUST NOT be used in descriptors specified in other OCI specifications:
 
 - **`data`** *string*