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/index.rst b/doc/develop/bootstd/index.rst
index f8fce72..69fd3c2 100644
--- a/doc/develop/bootstd/index.rst
+++ b/doc/develop/bootstd/index.rst
@@ -10,3 +10,4 @@
    extlinux
    pxelinux
    qfw
+   cros