Add info how to generate artifact file

[pawurb]

Hi. For me it wasn't at first obvious what format of the file is needed. I think adding this explanation will be helpful.

[DaniPopes]

Good catch! It's either a JSON ABI array, or an artifact from hardhat/forge, mainly an object with "abi" as a field.

The example here is worded weirdly since it mentions "Solidity file" while showing a JSON file path; you can check docs.rs for the full macro docs.

[zerosnacks]

Thanks!