Write project README

This repository needs a README which explains the project's use in the context of out-of-tree builds, as well as instructions on how to get started, links to crates.io/docs.rs, etc.

Start at the repository root and inspect the Rust crate metadata, existing documentation, and out-of-tree build setup. Create a README covering out-of-tree use, getting started, and links to crates.io and docs.rs; done means each requested topic is explained and the links are present.

Written by the indexing model from the issue text.