docs: add readme

README.md

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+# xMnt
+
+A helper for mounting encrypted partitions.
+
+## Use case
+
+I have encrypted partitions that I need to mount them from time to time.
+
+I use GPG to encrypt the decryption keys for each partition, save them as `${uuid}.key`
+
+With this little tool, I can mount and unmount them with one command.
+
+## Usage
+
+Presets are stored in "$XDG_CONFIG_HOME/xmnt/presets".
+
+For example, save the following content as `boot.yml` in the presets directory:
+
+```yaml
+type: blk
+path: /dev/nvme0n1p2
+mountpoint: /boot
+```
+
+Then when you run `xmnt boot`, the tool will decrypt `/dev/nvme0n1p2` and mount it on `/boot`.
+
+You can also use it without defining presets. See `--help` for detailed usage.
+
+## Supported partition types
+
+- luks encrypted partitions (e.g. ext4)
+- regular unencrypted partitions
+- zfs datasets