bootstd: doc: Add documentation

Add documentation for this feature, including the commands and full
devicetree bindings.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/distro.rst b/doc/develop/distro.rst
index 0113a3d..3ee3dac 100644
--- a/doc/develop/distro.rst
+++ b/doc/develop/distro.rst
@@ -157,6 +157,9 @@
 from Kconfig itself, for e.g. all boards using a specific SoC then
 add a "imply DISTRO_DEFAULTS" to your SoC CONFIG option.
 
+
+TO BE UPDATED:
+
 In your board configuration file, include the following::
 
     #ifndef CONFIG_SPL_BUILD