play: add readme

board/beagleplay/readme.txt

+BeagleBoard.org BeaglePlay
+
+Description
+===========
+
+This configuration will build an initramfs and kernel suitable
+for booting with existing u-boot on BeaglePlay eMMC.
+
+How to build it
+===============
+
+Select the default configuration for the target:
+$ make beagleplay_defconfig
+
+Optional: modify the configuration:
+$ make menuconfig
+
+Build:
+$ make
+
+Result of the build
+===================
+
+$ tree output/images/
+output/images/
+├── Image
+├── rootfs.cpio
+├── rootfs.cpio.gz
+├── rootfs.tar
+└── ti-connectivity
+    ├── TIInit_7.2.31.bts
+    ├── wl1271-nvs.bin -> wl127x-nvs.bin
+    ├── wl127x-nvs.bin
+    ├── wl12xx-nvs.bin -> wl127x-nvs.bin
+    ├── wl18xx-fw-2.bin
+    ├── wl18xx-fw-3.bin
+    ├── wl18xx-fw-4.bin
+    └── wl18xx-fw.bin
+
+Booting the build
+=================
+
+To boot this image on a board with an existing u-boot+Debian setup, 
+
+```
+# mount /dev/mmcblk0p1 /mnt
+# mv /mnt/Image /mnt/Image-old
+# cp Image /mnt/Image
+# mv /mnt/initrd.img /mnt/initrd.img-old
+# cp rootfs.cpio.gz /mnt/initrd.img
+# sync
+# reboot
+```
+
+Over serial, login with username `root` and password `temppwd`.
+
+Tested hardware
+===============
+
+BeaglePlay (rev. A2)
+
+2023, Jason Kridner <jkridner@beagleboard.org>