Add README

README.md

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+# Pico SDR
+
+Using RP2040 / Raspberry Pi Pico as a software-defined radio receiver.
+
+1. Clone using `git clone --recursive` as this package is using a custom USB
+   stdio library for better throughput and to avoid deadlocks.
+
+2. Build and flash the firmware as usual:
+
+   ```bash
+   export PICO_SDK_PATH=/path/to/pico-sdk
+   cmake -B build src
+   cmake --build build
+   picotool load -f build/pico_sdr.uf2
+   ```
+
+3. Start the USB serial to TCP bridge, setting the frequency to 88.2 MHz:
+
+   ```bash
+   python util/bridge.py -f 88200000
+   ```
+
+4. Open `grc/PicoSDR-WBFM.grc` in GNU Radio Companion.
+
+5. Press `F6`.