doc: dfu: tftp: README entry for TFTP extension of DFU

Documentation file for DFU extension. With this functionality it is now
possible to transfer FIT images with firmware updates via TFTP and use
DFU backend for storing them.

Signed-off-by: Lukasz Majewski <l.majewski@majess.pl>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/doc/README.update b/doc/README.update
index a7f4d9e..eab124c 100644
--- a/doc/README.update
+++ b/doc/README.update
@@ -93,3 +93,10 @@
   An example containing three updates. It can be used to update Linux kernel,
   ramdisk and FDT blob stored in Flash. The procedure for preparing the update
   file is similar to the example above.
+
+TFTP update via DFU
+-------------------
+
+- It is now possible to update firmware (bootloader, kernel, rootfs, etc.) via
+  TFTP by using DFU (Device Firmware Upgrade). More information can be found in
+  ./doc/README.dfutftp documentation entry.