Either a README section, or a CONTRIBUTING.md is needed to explain to people what they need in terms of software in order to build and test subplot. Ideally it'd also contain our Minimum-Supported-Rust-Version (MSRV) policy and details of how to set up to validate that too. For every backend language we support we need details of verifying that backend as well as details of what to do to verify the document outputs since that's harder to automate.