Document console output differences across development modes

Add clear guidance about expected console warnings in different dev modes:

- `bin/dev` (HMR): Shows HMR + resource preload warnings (normal)
- `bin/dev static`: Shows only resource preload warnings (cleaner)
- `bin/dev prod`: Cleanest output, minimal warnings (production-like)

Explains that resource preload warnings are normal in development due to
Shakapacker's preload tag generation with async script loading. This helps
contributors understand which warnings to ignore vs. investigate.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: justin808

CONTRIBUTING.md

@@ -304,7 +304,13 @@ bin/dev
 - Visit the hello_world page in browser
 - Check browser console for "RENDERED HelloWorld to dom node" success message
 - Confirm React component is interactive (input field updates name display)
-- Use `bin/dev static` mode for cleanest console output during testing
+
+**Development Mode Console Output**:
+- `bin/dev` (HMR): Shows HMR warnings and resource preload warnings (expected)
+- `bin/dev static`: Shows only resource preload warnings (cleaner output)
+- `bin/dev prod`: Cleanest output with minimal warnings (production-like environment)
+
+**Note**: Resource preload warnings in development modes are normal and can be ignored. They occur because Shakapacker generates preload tags but scripts load asynchronously. Production mode eliminates most of these warnings.
 
 ### Linting