Clarify that "Image Layout" is OPTIONAL

The current image spec explicitly says that "This layout MAY be used in a variety of different transport mechanisms" (image-layout.md).

However, it doesn't explicitly say that "This layout is OPTIONAL" (regardless to the transportation).

Actually, I was rather misunderstanding that using the layout were "REQUIRED but not specific to a certain transport mechanism", and got confused for that index.json is REQUIRED (image-layout.md) for layout but OPTIONAL (image-index.md)for image providers. (#716, #719)

So I'd like to open a PR as follows:

diff --git a/image-layout.md b/image-layout.md
index 0c762c1..5351150 100644
--- a/image-layout.md
+++ b/image-layout.md
@@ -1,6 +1,7 @@
 ## OCI Image Layout Specification

 * The OCI Image Layout is a slash separated layout of OCI content-addressable blobs and [location-addressable](https://en.wikipedia.org/wiki/Content-addressable_storage#Content-addressed_vs._location-addressed) references (refs).
+* Conformity to this layout specification is OPTIONAL unless an image contains `oci-layout` file.
 * This layout MAY be used in a variety of different transport mechanisms: archive formats (e.g. tar, zip), shared filesystem environments (e.g. nfs), or networked file fetching (e.g. http, ftp, rsync).

 Given an image layout and a ref, a tool can create an [OCI Runtime Specification bundle](https://github.com/opencontainers/runtime-spec/blob/v1.0.0/bundle.md) by:

Start by reading image-layout.md and the related image-index.md references cited in the issue. Check whether the proposed clarification matches the existing distinction between layouts and image providers; done when the specification clearly states the intended optionality without creating a contradiction.