docs: add a readme to the pi pico example

examples/rp-pico/README.md

@@ -0,0 +1,28 @@
+# Using the AHT20 with a Raspberry Pi Pico
+
+![Photo of AHT20 Adafruit board with a Raspberry Pi Pico](breadboard_photo.jpg)
+
+
+![Breadboard diagram with the AHT20 and Pi Pico](bradboard_diagram.png)
+
+This example relies on running the example using a debug probe, such as the
+[Raspberry Pi debug
+probe](https://www.raspberrypi.com/documentation/microcontrollers/debug-probe.html).
+The example uses [probe-rs](https://probe.rs/) to flash your Rasbperry Pi Pico
+and to get defmt log messages transmitted and displayed. If you have your probe
+set up and connected, then all you need to run this example is to run:
+
+```
+DEFMT_LOG=info cargo run
+```
+
+You should then start to get output like:
+
+```
+INFO  setup done
+└─ aht20_rp_pico_example::__cortex_m_rt_main @ src/main.rs:106
+INFO  temperature: 22.808838
+└─ aht20_rp_pico_example::__cortex_m_rt_main @ src/main.rs:116
+INFO  humidity: 50.65298
+└─ aht20_rp_pico_example::__cortex_m_rt_main @ src/main.rs:117
+```