Document How to Script and Contribute Custom Mux Modes

[ruffsl]

Summary:
Encourage developer involvement and upstream contributions by documenting how users can create and contribute their own custom mux modes. The process is straightforward for those with basic Rust knowledge and a working development environment.

How to Create a Custom Mux Mode:

1. Create a new source file in the src/mux_modes/ directory.
2. Define a new struct implementing the MuxMode trait, specifically the handle_event method.
3. Update mod.rs to:
   • Import the new module
   • Add the new mode to the ModeType enum
   • Extend the factory function to dispatch the new mode
4. Rebuild and run CtrlAssist, selecting your new mode via the CLI.

Advanced Ideas:

• Explore forwarding additional CLI arguments to custom mux modes (e.g., ctrlassist mux --mode toggle -- --button=select to override the default toggle button).
• Consider using clap subcommand or argument injection for mode-specific configuration.
• Document best practices for structuring, testing, and submitting new mux modes upstream.

Notes:

• Lowering the barrier for scripting and contributing mux modes will foster innovation and community involvement.
• Example templates and documentation should be provided to guide new contributors.