Document the Minimum Supported Rust Version (MSRV)

[louib]

I don't think we currently document the minimum version of Rust supported by the library. There is now a Cargo field that we can use to document this requirement. There's also a crate that can allow us to find the current minimum version we support. I'm thinking we could eventually use this crate to detect in CI if the MSRV changes. This will allow us to make an explicit decision when bumping the minimum version.