mb2-template: MicroBit 2 Rust embedded project template

To use this Github template repository:

1. Follow the instructions linked above to make a Github repo for your project.

2. If needed, do the following to set up your tools:

   rustup target add thumbv7em-none-eabihf rustup component add llvm-tools cargo install cargo-binutils cargo install probe-rs --features cli

3. Edit this README.md, the Cargo.toml and the stuff in src/ to get the names right and the template to what you need.