Clarify that this version only uses OP_RETURN

[Sjors]

If I understand correctly the current version only implements the OP_RETURN commitment scheme and not yet the Pay-to-contract one?

That would also explain why the Contract constructor doesn't have a commitment_scheme field.

From contract.rs:

    fn get_expected_script(&self) -> Script {
        let burn_script_builder = Builder::new();

        let burn_script_builder = burn_script_builder.push_opcode(All::OP_RETURN);
        let burn_script_builder = burn_script_builder.push_slice(self.bitcoin_hash().as_bytes());

        burn_script_builder.into_script()
    }

Is burn_script_builder a typo? It's seems to be called from places that have nothing to do with burn.

I find it confusing that the function get_expected_script lives under Verify for Contract, even though it's also used when constructing proof in kaleidoscope tx_builder.ts.

In general it's good to explicitly state which parts of the spec are (not) implemented.