Add canister ID to README

[peterpeterparker]

What's the canister ID of the chain-fusion-signer? It would be nice to provide this information in the README.

[bitdivine]

It is in the canister_ids.json, as is standard for dfx projects. That said, the canister ID is very unlikely to change so there is little risk of a value in README.md going out of date.

I have also been asked for examples of use. I could add an example including the canister ID.

[bitdivine]

Done: #241