Document factory deployment (with foundry)

[fmnxl]

Document how the CREATE3Factory itself should be deployed on different chains so that it deploys contracts to the same addresses.

e.g. for foundry scripts

function run() public {
  vm.startBroadcast();

  // Keep the same salt across different chains
  CREATE3Factory factory = new CREATE3Factory{salt: "MY_CREATE3_FACTORY"}();
  
  factory.deploy(...);
  ...

I believe the deployed address of the factory itself matters? Correct me if I'm wrong.