Move checkpoint/restore readme to g3doc directory.

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+# runsc checkpoint/restore
+
+gVisor supports checkpointing and restoring containers. A container’s state can
+be checkpointed and later restored into one or more containers. This can be used
+to save work and time in cases of failure and allow for container migration. A
+single container can perform slower setup tasks and then be checkpointed so that
+many containers with the same task can be “restored” and started more quickly.
+
+### How to checkpoint/restore
+
+To use the runsc checkpoint command, first run a container.
+
+```sh
+runsc run <container id>
+```
+
+To checkpoint the container, the --image-path flag must be provided. This is the
+directory path within which the checkpoint state-file will be created. The file
+will be called checkpoint.img and necessary directories will be created if they
+do not yet exist.
+
+> Note: Two checkpoints cannot be saved to the save directory; every image-path
+provided must be unique.
+
+```sh
+runsc checkpoint --image-path=<path> <container id>
+```
+
+There is also an optional --leave-running flag that allows the container to
+continue to run after the checkpoint has been made. (By default, containers stop
+their processes after committing a checkpoint.)
+
+> Note: All top-level runsc flags needed when calling run must be provided to
+checkpoint if --leave-running is used.
+
+> Note: --leave-running functions by causing an immediate restore so the
+container, although will maintain its given container id, may have a different
+process id.
+
+```sh
+runsc checkpoint --image-path=<path> --leave-running <container id>
+```
+
+To restore, provide the image path to the checkpoint.img file created during the
+checkpoint. Because containers stop by default after checkpointing, restore
+needs to happen in a new container (restore is a command which parallels start).
+
+```sh
+runsc create <container id>
+
+runsc restore --image-path=<path> <container id>
+```
+
+### How to use checkpoint/restore in Docker:
+
+Currently checkpoint/restore through runsc is not entirely compatible with
+Docker, although there has been progress made from both gVisor and Docker to
+enable compatibility. Here, we document the ideal workflow.
+
+To run with Docker, first follow the [instructions](https://gvisor.googlesource.com/gvisor/+/master/README.md#configuring-docker) to use runsc as a runtime.
+
+Run a container:
+
+```sh
+docker run [options] --runtime=runsc <image>`
+```
+
+Checkpoint a container:
+
+```sh
+docker checkpoint create <container> <checkpoint_name>`
+```
+
+Create a new container into which to restore:
+
+```sh
+docker create [options] --runtime=runsc <image>
+```
+
+Restore a container:
+
+```sh
+docker start --checkpoint --checkpoint-dir=<directory> <container>
+```
+
+**Issues Preventing Compatibility with Docker**
+1. [Moby #37360][leave-running]
+
+Docker version 18.03.0-ce and earlier hangs when checkpointing and
+does not create the checkpoint. To successfully use this feature, install a
+custom version of docker-ce from the moby repository. This issue is caused by an
+improper implementation of the `--leave-running` flag. This issue is now fixed
+although is not yet part of an official release.
+
+2. Docker does not support restoration into new containers.
+
+Docker currently expects the container which created the checkpoint
+to be the same container used to restore which is not possible in runsc. When
+Docker supports container migration and therefore restoration into new
+containers, this will be the flow.
+
+3. [Moby #37344][checkpoint-dir]
+
+Docker does not currently support the `--checkpoint-dir` flag but this will be
+required when restoring from a checkpoint made in another container.
+
+[leave-running]: https://github.com/moby/moby/pull/37360
+[checkpoint-dir]: https://github.com/moby/moby/issues/37344