fix: improve links in README

Author: hsluoyz

README.md

@@ -239,28 +239,17 @@ https://casbin.org/docs/adopters
 This project exists thanks to all the people who contribute. 
 <p><a href="https://github.com/casbin/Casbin.NET/graphs/contributors"><img src="https://opencollective.com/casbinnet/contributors.svg?width=890&button=false"></a></p>
 
-## Backers
+## Star History
 
-Thank you to all our backers! 🙏 [[Become a backer](https://opencollective.com/casbin#backer)]
+[![Star History Chart](https://api.star-history.com/svg?repos=casbin/Casbin.NET&type=Date)](https://star-history.com/#casbin/Casbin.NET&Date)
 
-<p><a href="https://opencollective.com/casbin#backers" target="_blank"><img src="https://opencollective.com/casbin/backers.svg?width=890"></a></p>
+## License
 
-## Sponsors
+This project is licensed under the [Apache 2.0 license](LICENSE).
 
-Support this project by becoming a sponsor. Your logo will show up here with a link to your website. [[Become a sponsor](https://opencollective.com/casbin#sponsor)]
+## Contact
 
-<p><a href="https://opencollective.com/casbin/sponsor/0/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/0/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/1/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/1/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/2/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/2/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/3/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/3/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/4/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/4/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/5/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/5/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/6/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/6/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/7/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/7/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/8/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/8/avatar.svg"></a>
-<a href="https://opencollective.com/casbin/sponsor/9/website" target="_blank"><img src="https://opencollective.com/casbin/sponsor/9/avatar.svg"></a>
-</p>
+If you have any issues or feature requests, please contact us. PR is welcomed.
+- https://github.com/casbin/Casbin.NET/issues
+- https://discord.gg/S5UjpzGZjN
 
-## License
-
-This project is licensed under the [Apache 2.0 license](LICENSE).