docs: add readme for ds18b20 demos

demo/gpio/ds18b20/README.md

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+# DS18B20
+
+A very popular digital thermometer, temperature monitoring with 2-wire or 3-wire Connections.
+
+# Wiring
+
+## Normal Power Mode
+
+3-wire connection of normal power mode
+
+* P35         -> DQ
+* GND         -> GND
+* 5V/3.3V     -> VDD
+
+## Parasite Power Mode:
+
+In parasite power mode, it's a 2-wire connection. It requires both DS18B20 GND and Vdd to be connected to ground. The DQ pin is the data/parasite power line, which requires a pull-up resistor (set by PxPU command).
+
+It has to be noted that **not** all DS18B20 you bought online works in parasite mode. There are cheap units that output `0` only in parasite mode, in this case you need some modifications to make it work.
+
+* P35   -> DQ
+* GND   -> GND -> VDD
+* 5V
+
+## Parasite Power Mode Emulation:
+
+In case some DS18B20 doesn't work in parasite mode, you can add one 0.1uF capacitor and one 1N4148 to achieve 2-wire connection. 
+In thise way DS18B20 actually works in normal power mode
+
+```
+     +-----1N4148-|>|-----+
+     |                    |
+     |     |DS18B20|-VCC--+
+     |     |       |      |
+P35 --+-DQ--|DS18B20|     0.1uF
+           |       |      |
+GND ----GND-|DS18B20|-GND--+
+```
+