liboci-cli: Update README with information about features subcommand

The `features` subcommand is  implemented in `runc`, but not documented.
See opencontainers/runc#3296

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>
Suggested-by: Toru Komatsu <k0ma@utam0k.jp>

Author: c3d

crates/liboci-cli/README.md

@@ -16,6 +16,7 @@ Interface](https://github.com/opencontainers/runtime-tools/blob/master/docs/comm
 | checkpoint |            |                   | ✅   | ✅   |       |
 | events     | ✅         |                   | ✅   |      | ✅    |
 | exec       | ✅         |                   | ✅   | ✅   | ✅    |
+| features   | ✅         |                   | 🏴   |     |      |
 | list       | ✅         |                   | ✅   | ✅   | ✅    |
 | pause      | ✅         |                   | ✅   | ✅   | ✅    |
 | ps         | ✅         |                   | ✅   | ✅   | ✅    |
@@ -24,3 +25,6 @@ Interface](https://github.com/opencontainers/runtime-tools/blob/master/docs/comm
 | run        | ✅         |                   | ✅   | ✅   | ✅    |
 | spec       | ✅         |                   | ✅   | ✅   | ✅    |
 | update     |            |                   | ✅   | ✅   |       |
+
+🏴 The `features` subcommand is [implemented](https://github.com/opencontainers/runc/pull/3296)
+in `runc`, but not documented.