adds some description re: compat with docker schemas

Signed-off-by: Mike Brown <brownwm@us.ibm.com>

manifest.md

@@ -22,7 +22,9 @@ A client will distinguish a manifest list from an image manifest based on the Co
 - **`mediaType`** *string*
 
   This REQUIRED property contains the MIME type of the manifest list.
-  For this version of the specification, this MUST be set to `application/vnd.oci.image.manifest.list.v1+json`.
+  For this version of the specification, this MUST be set to
+  `application/vnd.oci.image.manifest.list.v1+json`. For the MIME type that this
+  is compatible with see the [matrix](media-types.md#compatibility-matrix).
 
 - **`manifests`** *array*
 
@@ -123,7 +125,9 @@ Unlike the [Manifest List](#manifest-list), which contains information about a s
 - **`mediaType`** *string*
 
     This REQUIRED property contains the MIME type of the image manifest.
-    For this version of the specification, this MUST be set to `application/vnd.oci.image.manifest.v1+json`.
+    For this version of the specification, this MUST be set to
+    `application/vnd.oci.image.manifest.v1+json`. For the MIME type that this is
+    compatible with see the [matrix](media-types.md#compatibility-matrix).
 
 - **`config`** *[descriptor](descriptor.md)*