README note

[benatkin]

Perhaps a README note saying this can be done by the host would be good, for people happening upon this because they want to use SHA256. SHA256 is pretty ubiquitious in WebCrypto now, and can be used in WASM through wasm_import_module on Rust, and other ways on other languages:

https://rustwasm.github.io/docs/book/reference/js-ffi.html#from-the-rust-side