Document RBS type signature support in README

[justin808]

Summary

Add documentation about RBS type signature support to help developers leverage type checking in their IDEs.

Motivation

React on Rails now has comprehensive RBS type signatures (added in PR #1945), but developers may not be aware of this feature or how to use it.

Proposed Changes

Add a new section to README.md:

Suggested Content

## Type Safety (RBS)

React on Rails includes [RBS](https://github.com/ruby/rbs) type signatures for improved type safety and IDE support.

### Benefits
- Better autocomplete in supported IDEs
- Early detection of type errors
- Improved documentation through types

### IDE Support
RBS signatures work with:
- [Steep](https://github.com/soutaro/steep) - Type checker for Ruby
- [Solargraph](https://solargraph.org/) - Ruby language server with RBS support
- RubyMine (built-in RBS support)

### Validation
To validate type signatures:
\`\`\`bash
bundle exec rbs -I sig validate
# or
rake rbs:validate
\`\`\`

### Location
Type signatures are located in `sig/react_on_rails/`

Additional Documentation

Consider also adding:

• Link to RBS documentation
• Version compatibility notes (which Ruby/RBS versions are supported)
• Examples of how types improve the development experience

Related

Follow-up to PR #1945 - post-merge code review feedback