devmapper: add README and minor fixes

Signed-off-by: Maksym Pavlenko <makpav@amazon.com>

snapshots/devmapper/README.md

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+## Devmapper snapshotter
+
+Devmapper is a `containerd` snapshotter plugin that stores snapshots in ext4-formatted filesystem images
+in a devicemapper thin pool.
+
+## Setup
+
+To make it work you need to prepare `thin-pool` in advance and update containerd's configuration file.
+This file is typically located at `/etc/containerd/config.toml`.
+
+Here's minimal sample entry that can be made in the configuration file:
+
+```
+[plugins]
+  ...
+  [plugins.devmapper]
+    pool_name = "containerd-pool"
+    base_image_size = "128MB"
+  ...
+```
+
+The following configuration flags are supported:
+* `root_path` - a directory where the metadata will be available (if empty
+  default location for `containerd` plugins will be used)
+* `pool_name` - a name to use for the devicemapper thin pool. Pool name
+  should be the same as in `/dev/mapper/` directory
+* `data_device` - path to the data volume that should be used by the thin pool
+* `meta_device` - path to the metadata volume that should be used by the thin-pool
+* `data_block_size` - the size of allocation chunks in data file, between 128
+  sectors (64KB) and and 2097152 sectors (1GB) and a multiple of 128 sectors (64KB)
+* `base_image_size` - defines how much space to allocate when creating the base device
+
+Pool name and base image size are required snapshotter parameters.
+
+## Run
+Give it a try with the following commands:
+
+```bash
+ctr images pull --snapshotter devmapper docker.io/library/hello-world:latest
+ctr run --snapshotter devmapper docker.io/library/hello-world:latest test
+```
+
+## Requirements
+
+The devicemapper snapshotter requires `dmsetup` command line tool to be installed and available on your computer.
+On Ubuntu, it can be installed with `apt-get install dmsetup` command.