[docs] plugins-tutorial: reference the amper-plugins-tutorial repo

Author: klikh

docs/src/user-guide/plugins/quick-start.md

@@ -9,6 +9,9 @@ Our plugin should be able to parse a `.properties` file and generate Kotlin prop
 Later we may implement additional features.
 We will name our plugin `build-config`.
 
+The sources of the plugin are available in the [amper-plugins-tutorial](https://github.com/JetBrains/amper-plugins-tutorial)
+repository on GitHub. You can clone it, checkout the initial revision, and follow the Git log to see the changes made step by step.
+
 ### Basic example
 
 Let's start with the following project structure: