Add README

README.md

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+# Pico Environment Sensor
+
+This firmware, written in Rust using embedded-hal ([rp235x-hal](https://crates.io/crates/rp235x-hal)), is designed for the [Raspberry Pi Pico 2 W](https://www.raspberrypi.com/documentation/microcontrollers/pico-series.html#pico2w-technical-specification) platform using the RP2350 microcontroller. Its application is an environment-sensing device which can measure CO2 concentration in the atmosphere, ambient temperature and humidity. 
+
+## Building
+
+### Prerequisites
+
+- Rust compiler & cargo package manager: https://www.rust-lang.org/
+- RPi Pico SDK: https://github.com/raspberrypi/pico-sdk
+- RPi picotool: https://github.com/raspberrypi/picotool
+- ARM bare metal compiler toolchain
+    - `arm-none-eabi-gcc` (& `arm-none-eabi-newlib`) on Arch Linux, your system may have different package names
+
+### Environment configuration
+
+This build system assumes the environment variables: `PICO_BOARD`, `PICO_PLATFORM` & `PICO_SDK_PATH` are set. `picotool` should also be available in the `PATH`.
+
+The [env-vars.sh](./scripts/env-vars.sh) script sets these values to `pico2_w`, `rp2350-arm-s` and my personal Pico SDK path, respectively.
+
+### Compiling & running
+
+- `cargo build` (or `cargo build --release`)
+
+If you wish to run the binary on your Pico (connected in BOOTSEL mode):
+
+- `cargo run` (or `cargo run --release`)