add README

README.md

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+# Synopsis
+
+Exposes readings from an ADC pin (currently: *PA3*) of the board via a
+TCP service on the Ethernet port.
+
+
+# Network Protocol
+
+Sensor readings produce lines of `key=value` pairs, joined by `,`,
+terminated by `"\r\n"`.
+
+```
+t=21000,pa3=685
+t=22000,pa3=684
+t=23000,pa3=681
+t=24000,pa3=696
+t=25000,pa3=673
+t=26000,pa3=689
+t=27000,pa3=657
+t=28000,pa3=654
+t=29000,pa3=652
+t=30000,pa3=662
+t=31000,pa3=663
+```
+
+| Key | Value       | Unit |
+|:---:|-------------|------|
+| t   | Time        | ms   |
+| pa3 | ADC reading | mV   |
+
+
+# LEDs
+
+Colors indicate what the MCU is occupied with.
+
+| Color   | Indication        |
+|:-------:|-------------------|
+| Green   | WFI (idle)        |
+| Blue    | Network poll      |
+| Red     | Message broadcast |
+
+
+# Crate features
+
+* `semihosting` enables log output via the **cortex-m-semihosting**
+  crate. Use only in development! MCU will hang when no OpenOCD is
+  running.
+
+* `generate-hwaddr` generates an Ethernet MAC address by hashing the
+  unique device ID from flash memory.
+
+
+# Instructions
+
+![Made for NixOS](https://nixos.org/logo/nixos-lores.png)
+
+## Build the firmware with `default.nix`
+
+* `nix-build`
+* This uses **cargo-vendor** to bundle dependencies, so that unstable versions from git can be used.
+* Run `result/bin/flash-adc2tcp` to flash a devboard with OpenOCD and quit.
+
+## Development environment with `shell.nix`
+
+* `nix-shell`
+* Spawning `openocd`, the devboard should be connected already.
+* Instructions (`cargo run --release`) are printed.