README.md: Add high-level link to optional image-layout

To make it clear that supporting image-layout is optional, because
there's no way to test support without requiring a particular backend
(tar, HTTP, etc.).

Also adjust the hashing wording folling Jonathan's suggestion [1] so
we can justify a link to descriptor.md while staying close to the
current scope-table wording.

[1]: opencontainers#380 (comment)

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

README.md

@@ -40,7 +40,8 @@ The [OCI Image Media Types](media-types.md) document is a starting point to unde
 The high-level components of the spec include:
 
 * An archival format for container images, consisting of an [image manifest](manifest.md), an [image layout](image-layout.md), a set of [filesystem layers](layer.md), and [image configuration](config.md) (base OCI layer)
-* A process of hashing the image format for integrity and content-addressing (base OCI layer)
+* A [process of referencing container images by a cryptographic hash of their content](descriptor.md) (base OCI layer)
+* A format for [storing CAS blobs and references to them](image-layout.md) (optional OCI layer)
 * Signatures that are based on signing image content address (optional OCI layer)
 * Naming that is federated based on DNS and can be delegated (optional OCI layer)