tegra: nyan: Add a README

Add a short note about how to boot U-Boot on Nyan-big using tegrarcm.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>

diff --git a/board/nvidia/nyan-big/README b/board/nvidia/nyan-big/README
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+# SPDX-License-Identifier: GPL-2.0+
+#
+# Copyright 2018 Google LLC
+# Written by Simon Glass <sjg@chromium.org>
+
+U-Boot on Nyan
+==============
+
+Nyan-big is supported by mainline U-Boot. This device is based on Tegra124.
+To build it, use the nyan-big config.
+
+To boot it, connect a USB A-A cable from your computer to the back USB port.
+Connect a servo board. Then with t20_rec and warn_rst held down:
+
+   sudo tegrarcm --bct cbootimage-configs/tegra124/nvidia/norrin/PM370_Hynix_2GB_H5TC4G63AFR_PBA_924MHz_01212014.bct
+           --bootloader u-boot-dtb-tegra.bin --loadaddr 0x80108000
+
+The norrin config is close enough that it works well with Nyan.