docs: add GitHub star badge to README footer

Author: NDXDeveloper

README.md

@@ -483,3 +483,15 @@ copies of the Software...
 ---
 
 **Why "Slashsum"?** Combination of "slash" (/) for file paths and "checksum" - because every good tool needs a catchy name!
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+---
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+<div align="center">
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+**Happy hashing!**
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+[![Star on GitHub](https://img.shields.io/github/stars/NDXDeveloper/slashsum?style=social)](https://github.com/NDXDeveloper/slashsum)
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+*If you find this useful, a star is always appreciated.*
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+</div>