Add --isolate option in README (#77)

Briefly explain in README the option to use the `--isolate` flag to execute some challenges.

Co-authored-by: tincho <[email protected]>

README.md

@@ -33,6 +33,9 @@ To solve a challenge:
 2. Uncover the flaw(s) in the challenge's smart contracts.
 3. Code your solution in the corresponding test file.
 4. Try your solution with `forge test --mp test/<challenge-name>/<ChallengeName>.t.sol`.
+
+> In challenges that restrict the number of transactions, you might need to run the test with the `--isolate` flag.
+
 If the test passes, you've solved the challenge!
 
 Challenges may have more than one possible solution.