Update README.md and translate CONTRIBUTING.md to English

[killerwolf]

Background

The project's documentation has two key issues that need to be addressed:

1. README.md contains minor inaccuracies and inconsistencies
2. CONTRIBUTING.md is currently in French and focused on maintainers rather than external contributors

Tasks for README.md

1. Correct the UMD path reference (currently inconsistent with actual output)
2. Update usage example that incorrectly uses new VisualImageTool.VisualImageTool()
3. Update the Vue example (currently using Vue 2 syntax)
4. Ensure the demo list is consistent with the actual demo directory contents
5. Fix script name inconsistency (format:write vs format:fix)
6. Add a note about default focus point behavior for clarity

Tasks for CONTRIBUTING.md

1. Translate the entire document from French to English
2. Refocus the content to guide external contributors rather than just maintainers
3. Include guidelines for:
   • Submitting issues
   • Creating pull requests
   • Following code style and coding standards
   • Running tests (once implemented)
   • Development workflow
4. Consider creating a separate MAINTAINER_GUIDE.md for maintainer-specific information

Expected Outcome

• Accurate and consistent README.md that properly guides users
• English CONTRIBUTING.md that encourages and guides external contributions
• Clear separation of maintainer vs contributor documentation

This addresses item #5 from the review in issue #6.