Add info about "WARNING GPIO4 is a Strapping PIN"

The current version of ESPHome shows a warning about GPIO4:
```
WARNING GPIO4 is a Strapping PIN and should be avoided.
Attaching external pullup/down resistors to strapping pins can cause unexpected failures.
See https://esphome.io/guides/faq.html#why-am-i-getting-a-warning-about-strapping-pins
```
The warning can be ignored, so I added a remark about this to the documentation.
Thanks to @jossuanet for the heads up about this warning.

doc/installation.md

@@ -19,6 +19,14 @@ found in the [Configuration guide](configuration.md).
 After these steps you can let ESPHome compile your firmware (`firmware.bin`) file. This firmware
 can then be flashed onto the device.
 
+**Note**: During compilation you might see a warning message from ESPHome, telling you:
+```
+WARNING GPIO4 is a Strapping PIN and should be avoided.
+```
+You can safely ignore this warning. GPIO4 is hard-wired in the lamp's PCB and the original firmware
+uses it as the master switch for turning on and off the LEDs. This way of using the pin does not
+pose any problems for the device.
+
 Like normal with ESPHome, the first time you will have to flash the device using a serial interface.
 After this initial flashing operation, you can flash new versions of the firmware using the OTA
 (Over The Air) method.