README: Add a table of contents
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]: https://github.com/opencontainers/image-spec/pull/304#issuecomment-246796088
     Subject: manifest: List media types supported by descriptor references

Signed-off-by: W. Trevor King <f75897670c17327c42ab201733fb8a81ff9f4c34@tremily.us>

opencontainers/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,vbatts/oci-image-spec,mattspencer-arm/image-spec,jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec