Document that the relay also powers the Nano

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/README.md

@@ -25,7 +25,8 @@ BROSE9323 driver is built with `-D ESPSERIAL`, so every GFX call is sent over `S
 - **Home Assistant** through MQTT (`home-assistant-integration` library) with two switches:
   - `FlipDotDisplayClock`: clock on, or the Nintendo logo when off
   - `FlipDotDisplayPower`: turns the display off through the relay
-- **Relay on GPIO 13** switches the display's power supply. `LOW` turns it on.
+- **Relay on GPIO 13** switches the power for both the panels and the Nano. `LOW` turns it
+  on. After switching on, the ESP waits 3 s so the Nano can boot before frames are sent.
 - **Status LED** (built-in) blinks fast when WiFi is down and slowly when connected. It is
   off at night or when the display is powered off.