Add README

config/boards/shields/rgbled_widget/README.md

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+# LED indicators using an RGB LED
+
+This is a shield containing a simple widget that utilizes a (typically built-in) RGB LED controlled by three separate GPIOs.
+
+By default this is supported by Xiao BLE and Xiao RP2040 boards, see overlays in the [`boards` folder](boards/).
+You can also add support for your board/shield by defining `aliases` to the RGB LED node labels, similar to the examples in that folder.
+
+Currently the widget does the following:
+- Blink green/yellow/red on boot depending on battery level (for both central/peripherals), thresholds set by `CONFIG_RGBLED_WIDGET_BATTERY_LEVEL_{HIGH,LOW}`
+- Blink red on every battery level read if below critical battery level (`CONFIG_RGBLED_WIDGET_BATTERY_LEVEL_CRITICAL`)
+- Blink blue for connected, yellow for open, red for disconnected profile on profile switch (on central side)
+- Blink blue for connection, red for disconnection on peripheral side
+
+Blink durations can also be adjusted, see the [Kconfig file](Kconfig.defconfig).