manifest: provide guidance on SCRATCH config descriptor

Through the conversations between artifact manifest and standardizing
the misuse of the image-manifest one of the topics was around when there
is _not_ a `config` needed for the `layers`/blobs.

Since the `config` is a REQUIRED field, it meant setting this to some
valid value.

This guidance intends to set a norm for a blob that need only be pushed
to a registry a single time, and then save on round trips for all future
SCRATCH configs, while also being most widely portability.

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

manifest.md

@@ -42,6 +42,12 @@ Unlike the [image index](image-index.md), which contains information about a set
 
         If the manifest uses a different media type than the above, it MUST comply with [RFC 6838][rfc6838], including the [naming requirements in its section 4.2][rfc6838-s4.2], and MAY be registered with [IANA][iana].
 
+    To set an effectively NULL or SCRATCH config and maintain portability the following is considered GUIDANCE.
+    While a 0 `size` may be preferable, practice has shown that not to be ubiquitious supported.
+    Instead, the blob payload can be the most minimal content that is still valid JSON: `{}` (size of 2).
+    The blob digest of `{}` is `sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a`.
+    See the [example SCRATCH config](#example-of-a-scratch-config-descriptor) below.
+
 - **`layers`** *array of objects*
 
     Each item in the array MUST be a [descriptor](descriptor.md).
@@ -122,6 +128,16 @@ Unlike the [image index](image-index.md), which contains information about a set
 }
 ```
 
+## Example of a SCRATCH config descriptor
+
+```json,title=SCRATCH%20config&mediatype=application/vnd.oci.descriptor.v1%2Bjson
+{
+  "mediaType": "application/vnd.oci.image.config.v1+json",
+  "size": 2,
+  "digest": "sha256:44136fa355b3678a1146ad16f7e8649e94fb4fc21fe77e8310c060f61caaff8a"
+}
+```
+
 [iana]:         https://www.iana.org/assignments/media-types/media-types.xhtml
 [rfc6838]:      https://tools.ietf.org/html/rfc6838
 [rfc6838-s4.2]: https://tools.ietf.org/html/rfc6838#section-4.2