[esp] Add I2C LCD example.

esp/cpp/06_i2c-scanner/README.md

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+# 06_i2c-scanner
+
+![schematic](./schematic.png)
+
+Simple I2C device scanner.
+
+For this example I used this [SSD1306 OLED display](https://www.digikey.com/en/products/detail/winstar-display/WEA012864DWPP3N00003/20533255).
+
+To build the example run the following commands.
+
+```bash
+source ~/path/to/esp-idf/export.sh
+
+mkdir build
+cd build
+cmake ..
+make -j $(nproc)
+
+# Flash to ESP32
+make flash
+
+# Open Serial Monitor, press CTRL+] to exit.
+make monitor
+```
+
+Expected output in serial monitor at 115200 baud
+
+```bash
+Scanning I2C devices...
+[0x3c]: Device found with clock rate 100000 and timeout 50
+Done.
+```