Add note on disk image partitioning in rpi docs (#331)

Signed-off-by: Fredrik Lönnegren <[email protected]>

docs/raspi-disk.md

@@ -51,3 +51,12 @@ Now we can write the `.raw` image to the SD-card. This can be done with `dd` on
 [openSUSE](https://www.opensuse.org) has nice instructions on how to write an image to a storage medium for [Linux](https://en.opensuse.org/SDB:Live_USB_stick),
 [Windows](https://en.opensuse.org/SDB:Create_a_Live_USB_stick_using_Windows), and [OS X](https://en.opensuse.org/SDB:Create_a_Live_USB_stick_using_macOS).
 
+### Starting the machine
+
+The raw disk image will only include the EFI partition, OEM partition and
+recovery partition. On first boot the system will boot into the recovery system
+to expand and add missing partitions. After expansion it will register with
+rancher and reboot.
+
+If an error occurs during registration phase the journal can be found using
+`journalctl -u elemental-register-reset`.