document new --from flag of vm.py

Author: zerosnacks

CONTRIBUTING.md

@@ -91,7 +91,13 @@ forge test -vvv
 
 Please follow the guide outlined in the [cheatcodes](https://github.com/foundry-rs/foundry/blob/master/docs/dev/cheatcodes.md#adding-a-new-cheatcode) documentation of Foundry.
 
-When making modifications to the native cheatcodes or adding new ones, please make sure to run [`./scripts/vm.py`](./scripts/vm.py) to update the cheatcodes in the [`src/Vm.sol`](./src/Vm.sol) file. This script will automatically generate the cheatcodes from the [`cheatcodes.json`](https://raw.githubusercontent.com/foundry-rs/foundry/master/crates/cheatcodes/assets/cheatcodes.json) file.
+When making modifications to the native cheatcodes or adding new ones, please make sure to run [`./scripts/vm.py`](./scripts/vm.py) to update the cheatcodes in the [`src/Vm.sol`](./src/Vm.sol) file.
+
+By default the script will automatically generate the cheatcodes from the [`cheatcodes.json`](https://raw.githubusercontent.com/foundry-rs/foundry/master/crates/cheatcodes/assets/cheatcodes.json) file, alternatively you can provide a path to a JSON file containing the Vm interface, as generated by Foundry, with the `--from` flag.
+
+```sh
+./scripts/vm.py --from path/to/cheatcodes.json
+```
 
 It is possible that the resulting [`src/Vm.sol`](./src/Vm.sol) file will have some changes that are not directly related to your changes, this is not a problem.