add more documentation

03-builder-pattern/README.md

@@ -49,3 +49,7 @@ note: `UnlockedCredentialManager::lock` takes ownership of the receiver `self`,
 36 |     pub fn lock(self) -> LockedCredentialManager {
    |                 ^^^^
 ```
+
+## Caveats
+
+This solution requires two structs for the same functionality which may hurt readability and conciseness of the code. Also, it requires to move data around between struct instances when transitioning between states which introduces unnecessary overhead and complexity.