Document MSRV

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Motivation

I'm adding a feature to wasm-bindgen (opening a PR soon), but don't know against which version of Rust I should check my changes. Clearly documenting the Minimum Supported Rust Version would help me test my changes.

Proposed Solution

As .github/workflows/main.yml uses Rust 1.56 for one of the tests, maybe we could start with this version? Then we could document it in the README, and/or by adding a rust-version field to Cargo.toml (which incidentally is supported on Rust 1.56 and up).

Alternatives

There may be other ways to document the MSRV, but "MSRV" itself has become a fairly common keyword to look for that information, so I'd like for it to be surfaced in the README and in Cargo.toml.

Additional Context

N/A.