Build instructions

I didn't see instructions for building (sorry if I missed them). Here's how I got it to work:

```
cargo install cargo-zigbuild
brew install zig
rustup target install aarch64-unknown-linux-gnu
cargo zigbuild --target aarch64-unknown-linux-gnu --release

# Binary is target/aarch64-unknown-linux-gnu/release/opensleep
```



Thanks for making this!