add current limitation to experimental/checkpoint-restore.md

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

experimental/checkpoint-restore.md

@@ -71,5 +71,17 @@ in between running/checkpoint/restoring you should see that the counter
 increases while the process is running, stops while it's checkpointed, and
 resumes from the point it left off once you restore.
 
-Note that seccomp is only supported by CRIU in very up to date kernels.
+## Current Limitation
+
+seccomp is only supported by CRIU in very up to date kernels.
+
+External terminal (i.e. `docker run -t ..`) is not supported at the moment.
+If you try to create a checkpoint for a container with an external terminal, 
+it would fail:
+
+    $ docker checkpoint create cr checkpoint1
+    Error response from daemon: Cannot checkpoint container c1: rpc error: code = 2 desc = exit status 1: "criu failed: type NOTIFY errno 0\nlog file: /var/lib/docker/containers/eb62ebdbf237ce1a8736d2ae3c7d88601fc0a50235b0ba767b559a1f3c5a600b/checkpoints/checkpoint1/criu.work/dump.log\n"
+    
+    $ cat /var/lib/docker/containers/eb62ebdbf237ce1a8736d2ae3c7d88601fc0a50235b0ba767b559a1f3c5a600b/checkpoints/checkpoint1/criu.work/dump.log
+    Error (mount.c:740): mnt: 126:./dev/console doesn't have a proper root mount