Add `features.md` and `features-linux.md`, to formalize the `runc features` JSON that was introduced in runc v1.1.0.
A runtime caller MAY use this JSON to detect the features implemented by the runtime.
The spec corresponds to https://github.com/opencontainers/runc/blob/v1.1.0/types/features/features.go
(opencontainers/runc PR 3296, opencontainers/runc PR 3310)
Differences since runc v1.1.0:
- Add `.linux.intelRdt.enabled` field
- Add `.linux.cgroup.rdma` field
- Add `.linux.seccomp.knownFlags` and `.linux.seccomp.supportedFlags` fields (Implemented in runc PR 3588)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>