Add README with pictures

Author: SimonSapin

README.md

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+I’ve previously played with
+and [written about](https://github.com/SimonSapin/rust-on-bbc-microbit#readme)
+Rust on [BBC micro:bit](http://microbit.co.uk/),
+but kind gave up on it because it’s somewhat difficult
+to make good electrical connection with the small pins on the edge connector
+and I was getting inconsistent brightness when driving LEDs
+directly from the microcontroller’s digital I/O pins
+and don’t know why.
+
+So I’ve switched to [Teensy](https://www.pjrc.com/teensy/teensy31.html)
+and got started with the
+[Bare Metal Rust on the Teensy 3.1](http://disconnected.systems/bare-metal-rust-on-the-teensy-3.1/)
+blog post.
+While not having any C/C++ code (only Rust and a linker script) seems attractive,
+it leaves you re-inverting the entire hardware abstraction layer,
+which is not so fun when doing stuff more involved than blinking a LED.
+
+So I want to use [Teensyduino](https://www.pjrc.com/teensy/teensyduino.html),
+the runtime software environment based on Arduino that everyone else uses for Teensy.
+But I don’t want to use the Arduino IDE.
+It turns out that the relevant code is available in
+the [PaulStoffregen/cores](https://github.com/PaulStoffregen/cores/) repository
+with a example Makefile that uses the `arm-none-eabi-gcc` toolchain.
+
+So far I’ve managed to:
+
+* Add a cross-compiled Rust static library into this mix
+* Use [Servo’s rust-bindgen](https://github.com/servo/rust-bindgen) (which has some C++ support)
+  to automatically generate Rust bindings for all of Teensyduino.
+* Have Rust code run on the Teensy and use these bindings for:
+  * USB serial with a Linux laptop
+  * i2c / TwoWire with an LED driver chip
+  * SPI with an RTC chip
+  * Setting up an interrupt on digital I/O input
+
+This is what’s in this repository.
+
+I’ve then tried to flip the build system around so that Rust makes an executable
+and the C/C++ code is a couple of static libraries built with the `gcc` crate (yay, no Makefile!)
+…but that doesn’t seem to boot and I don’t know why. (Yet, hopefully.)
+This can be found in [the `rustbuild` branch](
+https://github.com/SimonSapin/teensy-clock/compare/rustbuild).
+Any help with this is appreciated!
+
+The prototyping setup looks like this:
+
+<img src=pictures/proto.jpg height=500>
+
+And this is close to what I want to hang on my wall eventually.
+A clock that updates for summer time (a.k.a. daylight saving time) automatically.
+
+<img src=pictures/build1.jpg height=300>
+<img src=pictures/build2.jpg height=300>