add contributors reference in README.md (#442)

Author: Roberto Sora

README.md

@@ -16,6 +16,8 @@ Contributions are welcome!
 Please read the document [How to contribute](CONTRIBUTING.md) which will guide you through how to
 build the source code, run the tests, and contribute your changes to the project.
 
+:sparkles: Thanks to all our [contributors](https://github.com/arduino/arduino-cli/graphs/contributors)! :sparkles:
+
 ## How to install
 
 ### macOS/Linux installation via Homebrew