descriptor.md: add descriptions for the extended field

Signed-off-by: Qian Zhang <[email protected]>

Author: qianzhangxa

descriptor.md

@@ -43,6 +43,8 @@ The following fields contain the primary properties that constitute a Descriptor
 
     This OPTIONAL property contains arbitrary metadata for this descriptor.
     This OPTIONAL property MUST use the [annotation rules](annotations.md#rules).
+ 
+Descriptors pointing to [`application/vnd.oci.image.manifest.v1+json`](manifest.md) SHOULD include the extended field `platform`, see [Image Index Property Descriptions](https://github.com/qianzhangxa/image-spec/blob/master/image-index.md#image-index-property-descriptions) for details.
 
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