doc: Add booting instructions for Libreboot.

* doc/guix.texi (Booting): Mention the relevant command for USB.

Instructions for DVD and for Coreboot are missing.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -71,6 +71,7 @@ Copyright @copyright{} 2019 Alex Griffin@*
 Copyright @copyright{} 2019 Guillaume Le Vaillant@*
 Copyright @copyright{} 2020 Leo Prikler@*
 Copyright @copyright{} 2019, 2020 Simon Tournier@*
+Copyright @copyright{} 2020 Wiktor Żelazny@*
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -2004,6 +2005,8 @@ Access to @file{/dev/srX} usually requires root privileges.
 Once this is done, you should be able to reboot the system and boot from
 the USB stick or DVD.  The latter usually requires you to get in the
 BIOS or UEFI boot menu, where you can choose to boot from the USB stick.
+In order to boot from Libreboot, switch to the command mode by pressing
+the @kbd{c} key and type @command{search_grub usb}.
 @xref{Installing Guix in a VM}, if, instead, you would like to install
 Guix System in a virtual machine (VM).