update README

Author: martinbonnin

README.md

@@ -45,4 +45,20 @@ open class AbstractBoxTest : AbstractFirLightTreeBlackBoxCodegenTest() {
 }
 ```
 
-Kctf generates your test in `src/test/java`.
+Call the `kctfGenerateSources` task:
+
+```
+./gradlew kctfGenerateSources
+```
+
+Kctf generates your test in `src/test/java`:
+
+```java
+/** This class is generated by {@link kctf.MainKt}. DO NOT MODIFY MANUALLY */
+@SuppressWarnings("all")
+@TestMetadata("compiler-tests/src/test/data/diagnostic")
+@TestDataPath("$PROJECT_ROOT")
+public class DiagnosticTestGenerated extends AbstractDiagnosticTest {
+  // ...
+}
+```