[FEATURE] Add `LLMs.txt` to help AI agents understand the library

[ohprettyhak]

Is your feature request related to a problem? Please describe.
AI agents (like Cursor) don’t fully understand react-simplikit. Because of this, they sometimes ignore the existing functions in the library and instead generate new ones or look things up on the web.

Describe the solution you'd like
Adding an LLMs.txt file that gives clear context about the library, so AI agents can use the built-in functions more effectively.

Describe alternatives you've considered
Relying on documentation alone, but it doesn’t always help AI agents directly.

Additional context
This would improve DX and make AI-assisted workflows smoother when working with react-simplikit.