README: Add a table of contents

Linking the files from Makefile's DOC_FILES in their listed order.
Link text generally follows the headers used by the linked document.
Informative stuff (for image and implementation authors) is indented
under the README (although I'm not sure there's consensus on whether
media-types.md is normative or informative [1]).

[1]: #304 (comment)
     Subject: manifest: List media types supported by descriptor references

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

Author: wking

README.md

@@ -2,6 +2,20 @@
 
 The OCI Image Format project creates and maintains the software shipping container image format spec (OCI Image Format). The goal of this specification is to enable the creation of interoperable tools for building, transporting, and preparing a container image to run.
 
+## Table of Contents
+
+- [Introduction](README.md)
+    - [Code of Conduct](#code-of-conduct)
+    - [Project Documentation](project.md)
+    - [Media Types](media-types.md)
+- [Content Descriptors](descriptor.md)
+- [Image Layout](image-layout.md)
+- [Filesystem Layers](layer.md)
+- [Image Configuration](config.md)
+- [Manifests and Manifest Lists](manifest.md)
+
+## 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).
 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.