doc: Bring in the FIT overlay information

Bring this file into the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/fit/index.rst b/doc/usage/fit/index.rst
index 83bc4d0..bd25bd3 100644
--- a/doc/usage/fit/index.rst
+++ b/doc/usage/fit/index.rst
@@ -16,3 +16,4 @@
     signature
     verified-boot
     beaglebone_vboot
+    overlay-fdt-boot