doc: Add a description for bootmeth_cros

Add documentation for the cros bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
diff --git a/doc/develop/bootstd/overview.rst b/doc/develop/bootstd/overview.rst
index ef87896..de15fa4 100644
--- a/doc/develop/bootstd/overview.rst
+++ b/doc/develop/bootstd/overview.rst
@@ -421,6 +421,7 @@
    - U-Boot scripts from disk, network or SPI flash
    - EFI boot using bootefi from disk
    - VBE
+   - :doc:`ChromiumOS <cros>` ChromiumOS boot from a disk
    - EFI boot using boot manager
    - Android bootflow (boot image v4)
    - :doc:`QFW <qfw>`: QEMU firmware interface