[simple language plugin] reference on externalizing idea.home.path

code_samples/simple_language_plugin/build.gradle

@@ -18,6 +18,8 @@ test {
   // This path value is machine-specific placeholder text.
   // Set idea.home.path to the absolute path to the intellij-community source
   // on your local machine.
+  // Use variants described in https://docs.gradle.org/current/userguide/build_environment.html
+  // for real world projects.
   systemProperty "idea.home.path", "/Users/jhake/Documents/source/comm"
 }