EADME.md: add some description about manifest list

Signed-off-by: Lei Jitang <leijitang@huawei.com>

README.md

@@ -19,10 +19,11 @@ The goal of this specification is to enable the creation of interoperable tools
 
 ## Overview
 
-This specification defines how to create an OCI Image, which will generally be done by a build system, and output an [image manifest](manifest.md), a set of [filesystem layers](layer.md), and an [image configuration](config.md).
+This specification defines how to create an OCI Image, which will generally be done by a build system, and output an [image manifest](manifest.md), a [manifest list](manifest-list.md) (optional), a set of [filesystem layers](layer.md), and an [image configuration](config.md).
 At a high level the image manifest contains metadata about the contents and dependencies of the image including the content-addressable identity of one or more [filesystem layer changeset](layer.md) archives that will be unpacked to make up the final runnable filesystem.
 The image configuration includes information such as application arguments, environments, etc.
-The combination of the image manifest, image configuration, and one or more filesystem layers is called the "OCI Image".
+The manifest list is a higher-level manifest which points to specific image manifest for one or more platform.
+The combination of the manifest list (optional), image manifest, image configuration, and one or more filesystem layers is called the "OCI Image".
 
 The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC 2119](http://tools.ietf.org/html/rfc2119) (Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997).