feat(ui): add meaning of leds (#24)

content/en/docs/cover-ui-assembly/_index.md

@@ -24,3 +24,20 @@ After that, upload the cover UI firmware to Pico Board: https://github.com/Cleme
 You need `.uf2` file from `firmware.zip`. Hold `BOOT` button on Pico, insert USB and drag `.uf2` to Pico disk drive. 
 
 This is described in more details in [MicroPython guide](https://www.raspberrypi.com/documentation/microcontrollers/micropython.html#drag-and-drop-micropython), you can follow that, just use our `.uf2` file instead.
+
+## Meaning of LEDs and buttons
+
+Borrowed from Apehaenger's [custom firmware project](https://github.com/Apehaenger/CoverUI/tree/main/Firmware/CoverUI/yfc500#usage).
+
+* 2hr-8hr LEDs act as 4 digit progressbar for GPS quality. Blink = uncalibrated
+* Home button: Stop mowing and go back home to dock
+* Play button: Start mowing of recorded area(s)
+* S1: On = ROS is running but idle, Blink-slow = ROS in autonomous mode, Blink-fast = ROS in nirvana?
+* S2 LED: >TODO<
+* S2 Button: Short press, skip over to next mowing area. Long press (>= 3s), delete all recorded areas!
+* Lock Button: Clear emergency state (probably long press >=3s)
+* Mon-Sun as 7 digit progressbar for battery charge state (only in undocked state)
+* Lifted: Show emergency states. >TODO<
+* Wire: Also indicate a poor GPS. On = < 25%, Blink-fast = < 50%, Blink-slow < 75%, Off >= 75%
+* Battery: On = Battery empty
+* Charging LED: Fast blink = empty. Slow blink = approx. 1/2 charged. On = Fully charged.