Fix: update contribution guide link in README to local CONTRIBUTING.md (#4076)

Replaced the outdated external link to the contribution guide (which
returned 404) with a local reference to CONTRIBUTING.md. This ensures
contributors always have access to the latest guidelines without
encountering broken links.

Signed-off-by: MozirDmitriy <[email protected]>

Author: MozirDmitriy

README.md

@@ -146,4 +146,4 @@ We welcome contributions from the community! If you'd like to contribute to the
 4. Push to the branch (`git push origin feature/amazing-feature`)
 5. Open a Pull Request
 
-For detailed guidelines, see our [contribution guide](https://linera.dev/getting-started/contributing).
+For detailed guidelines, see our [contribution guide](./CONTRIBUTING.md).