update README with console.log details (gakonst#339)

* update README with console.log details

I found this useful, and figured it might be useful for others, so I gave it a quick writeup

* Apply suggestions from code review

Co-authored-by: Georgios Konstantopoulos <me@gakonst.com>

forge/README.md

@@ -210,6 +210,22 @@ interface Vm {
     // Expects an error on next call
     function expectRevert(bytes calldata) external;
 }
+```
+### `console.log`
+
+
+We support the logging functionality from Hardhat's `console.log`.
+
+If you are on a hardhat project, `import hardhat/console.sol` should just work if you use `forge test --hh`.
+
+If no, there is an implementation contract [here](https://github.com/gakonst/foundry/blob/master/evm-adapters/testdata/console.sol). We currently recommend that you copy this contract, place it in your `test` folder, and import it into the contract where you wish to use `console.log`, though there should be more streamlined functionality soon.
+
+Usage follows the same format as [Hardhat](https://hardhat.org/hardhat-network/reference/#console-log):
+```solidity
+import "./console.sol";
+...
+console.log(someValue);
+
 ```
 
 ## Future Features