Add docs/snapshots

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

docs/snapshotters/README.md

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+# Snapshotters
+
+Snapshotters manage the snapshots of the container filesystems.
+
+The available snapshotters can be inspected by running `ctr plugins ls` or `nerdctl info`.
+
+## Core snapshotter plugins
+
+Generic:
+- `overlayfs` (default): OverlayFS. This driver is akin to Docker/Moby's "overlay2" storage driver, but containerd's implementation is not called "overlay2".
+- `native`: Native file copying driver. Akin to Docker/Moby's "vfs" driver.
+
+Filesystem-specific:
+- `btrfs`: btrfs. Needs the plugin root (`/var/lib/containerd/io.containerd.snapshotter.v1.btrfs`) to be mounted as btrfs.
+- `zfs`: ZFS. Needs the plugin root (`/var/lib/containerd/io.containerd.snapshotter.v1.zfs`) to be mounted as ZFS. See also https://github.com/containerd/zfs .
+- `devmapper`: ext4/xfs device mapper. See [`devmapper.md`](./devmapper.md).
+
+[Deprecated](https://github.com/containerd/containerd/blob/main/RELEASES.md#deprecated-features):
+- `aufs`: AUFS. Deprecated since containerd 1.5. Planned to be removed in containerd 2.0. See also https://github.com/containerd/aufs .
+
+## Non-core snapshotter plugins
+
+- `fuse-overlayfs`: [FUSE-OverlayFS Snapshotter](https://github.com/containerd/fuse-overlayfs-snapshotter)
+- `nydus`: [Nydus Snapshotter](https://github.com/containerd/nydus-snapshotter)
+- `overlaybd`: [OverlayBD Snapshotter](https://github.com/containerd/accelerated-container-image)
+- `stargz`: [Stargz Snapshotter](https://github.com/containerd/stargz-snapshotter)