board/versal: add custom board documentation

The readme.txt file for versal boards does not have documentation for working
with custom boards.  This patch adds documentation for custom board support.

Signed-off-by: Neal Frager <neal.frager@amd.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

board/versal/readme.txt

@@ -60,3 +60,49 @@ card:
 Where 'sdX' is the device node of the SD.
 
 Eject the SD card, insert it in the board, and power it up.
+
+Support for other boards:
+=========================
+
+If you want to build a system for other boards based on the same SoC, and the
+board is already supported by the upstream kernel, U-Boot, and
+xilinx-prebuilt, you simply need to change the following Buildroot options:
+
+ - Kernel Device Tree file name (BR2_LINUX_KERNEL_INTREE_DTS_NAME)
+ - U-Boot (BR2_TARGET_UBOOT_CUSTOM_MAKEOPTS="DEVICE_TREE=<dts file name>")
+ - xilinx-prebuilt (BR2_TARGET_XILINX_PREBUILT_BOARD)
+
+Custom versal board support:
+
+To generate a boot.bin image, Versal boards require a PDI (Programmable Device
+Image) generated by Xilinx Vivado which contains all the hardware specific
+boot information, such as clock, MIO and DDR initializations as well as any
+customizations in the programmable logic. Since this PDI can only be generated
+by Xilinx Vivado, Buildroot needs access to the prebuilt image. The Buildroot
+xilinx-prebuilt package has support for Versal XSA files exported from Xilinx
+Vivado.
+
+Using the option BR2_TARGET_XILINX_PREBUILT_VERSAL_XSA, Buildroot can obtain
+the prebuilt PDI from the XSA file.
+
+1) Start with a defconfig supported by Buildroot (e.g. VCK190)
+    make versal_vck190_defconfig
+
+2) make menuconfig
+    Visit the following menu and enable BR2_TARGET_XILINX_PREBUILT_VERSAL_XSA
+
+    Bootloaders  --->
+        xilinx-prebuilt  --->
+            [*] download a prebuilt Versal XSA
+
+3) Within the same menuconfig, configure location of XSA. It can be in the
+   local file system or downloadable from an https:// location.
+
+    Bootloaders  --->
+        xilinx-prebuilt  --->
+            (<path to XSA>) URL of custom XSA
+
+4) make
+
+The resulting output/images will contain a boot.bin that includes the custom
+PDI file extracted from the Xilinx Vivado exported XSA file.