autoboot: Improve docs for CONFIG_AUTOBOOT_ENCRYPTION

This option is not documented properly at present. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/README b/README
index b08ad89..b493ffe 100644
--- a/README
+++ b/README
@@ -3421,6 +3421,8 @@
 		  allowed for use by the bootm command. See also "bootm_low"
 		  environment variable.
 
+  bootstopkeysha256, bootdelaykey, bootstopkey	- See README.autoboot
+
   updatefile	- Location of the software update file on a TFTP server, used
 		  by the automatic software update feature. Please refer to
 		  documentation in doc/README.update for more details.