Add Makefile

[jdolitsky]

I don't know much about rust, how do I build this thang?

[jlegrone]

1. Install rust tooling: https://doc.rust-lang.org/cargo/getting-started/installation.html
2. Clone repo and cargo build/cargo test: https://doc.rust-lang.org/cargo/guide/working-on-an-existing-project.html

cargo doc --open is also super useful. 😄

[radu-matei]

While most Rust projects seem to use Cargo to build and test, without going through a Makefile first, this should probably at least be documented.

Closing this issue, but opened #17 to track the documentation aspect - feel free to reopen if you think we should really add a Makefile.
Thanks!