docs: enhance contribution guidelines in README

Author: dido18

README.md

@@ -12,9 +12,16 @@
 For guidance on installation and development, see the [User documentation].
 
 ## Quickstart
+// TODO
 
 ## How to contribute
 
+Contributions are welcome!
+
+Please read the document [How to contribute] which will show you how to build the source code, run the tests, and
+contribute your changes to the project.
+
+:sparkles: Thanks to all our [contributors]! :sparkles:
 
 
 ## Security
@@ -37,3 +44,5 @@ license@arduino.cc
 [user documentation]:  https://github.com/arduino/arduino-app-cli/docs/user-documentation.md
 [how to contribute]: https://arduino.github.io/arduino-app-cli/latest/CONTRIBUTING/
 [security policy]: https://github.com/arduino/arduino-app-cli/security/policy
+[contributors]: https://github.com/arduino/arduino-cli/graphs/contributors
+