doc: add EDID I²C programming

README.md

@@ -17,6 +17,11 @@ if the ERROR LED is on, the possible cause is one of the following:
 - tried reading the EDID from the monitor, but it is not connected
 - communication with monitor failed, due to damaged cable
 - monitor EDID is invalid
+- storing EDID in EEPROM failed
+
+the firewall only acts as an I²C EEPROM at address 0x50 toward the HDMI device to provide the EDID information.
+if the EDID switch is on the BLOCK position, the EEPROM is read only.
+if the EDID switch is on the ALLOW position, writing the EEPROM is possible over the HDMI connection using standard I²C write operations.
 
 limitations
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