manifest: Require support for runtime-spec configurations

We probably need to keep application/vnd.oci.image.config.v1+json
untouched, since e94aa35 (schema: add a docker v2.2 backwards compat
test, 2016-06-15, #145) and other maintainer activity suggest a goal
of bit-for-bit compatibility with the current Docker schemas
(excepting media types).  However, requiring Docker support doesn't
mean we can't *also* require support for configuration formats that
are easier for image authors to use.

Of course, with the (greatly) increased flexibility comes a lot more
risk.  Image consumers in general, and runtime-spec-based-image
consumers in particular, should use a sanitization tool like [1].

The runtime-spec config also lacks support for diffIDs, but local
image tooling is still welcome to record the digests of uncompressed
layers and use that for local optimizations.  You have to fetch the
compressed layer at least once to perform the uncompression, but you'd
have to do that to verify the old diffID anyway.

[1]: opencontainers/runtime-tools#219

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

Author: wking

manifest.md

@@ -37,6 +37,7 @@ Unlike the [Manifest List](manifest-list.md), which contains information about a
         Implementations MUST support at least the following media types:
 
         - [`application/vnd.oci.image.config.v1+json`](config.md)
+        - [`application/vnd.oci.runtime.config.v1+json`](https://github.com/opencontainers/runtime-spec/blob/master/config.md), which MUST be unpacked to `config.json` without alteration.
 
         Manifests concerned with portability SHOULD use one of the above media types.