Enhance README with image and demo link

Added an image and demo link to the README.

Author: serkanyersen

README.md

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 DotState is a terminal-based tool that helps you manage your dotfiles effortlessly. Whether you're syncing your configuration across multiple machines or setting up a new development environment, DotState makes it simple, safe, and fast.
 
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 ## Demo
 
 https://github.com/user-attachments/assets/9be0df5e-87ce-4b61-ae0f-1c8ffe94cb36