doc: add cloud-init section

doc/usage.md

@@ -460,3 +460,33 @@ Proper use of this flag may look similar to the following section of a command:
 ```bash
 --device virtio-input,keyboard --device virtio-input,pointing --device virtio-gpu,width=1920,height=1080 --gui
 ```
+
+## Cloud-init
+
+#### Description
+
+The `-device cloud-init` option allows to supply an initial configuration which will be used by cloud-init to apply those settings when the instance is created.
+This option adds a disk to the virtual machine. The disk is backed by an image file on the host machine. This file is a ISO image file.
+
+The ISO image file must be labelled CIDATA and it must contain the user-data and meta-data files that will be used to modify the VM image on first boot. 
+It is also possible to add further configurations by using the network-config and vendor-data files.
+See https://cloudinit.readthedocs.io/en/latest/reference/datasources/nocloud.html#runtime-configurations for more details.
+
+To create the ISO image you could use this command within a folder containing the user-data and meta-data files
+```
+mkisofs -output seed.img -volid cidata -rock {user-data,meta-data}
+```
+
+See https://cloudinit.readthedocs.io/en/latest/reference/datasources/nocloud.html#example-creating-a-disk for further details about how to create a disk image
+
+#### Arguments
+- `path`: the absolute path to the disk image file.
+
+#### Example
+
+This adds cloud-init configuration to the guest:
+```
+--device cloud-init,path=seed.img
+```
+
+The `seed.img` file can be created with the tool/command above or any other equivalent tool/command.