[skip ci]add uml diagram

Author: raoul2000

guide/docs/class-ref.md

@@ -1 +1,7 @@
 Visit the [Class Reference Documentation](class-ref/index.html) for a complete description of the **yii2-workflow** implementation.
+
+## UML Diagram
+
+[view fullscreen](images/yii2-workflow-uml.jpg)
+
+![yii2-workflow UML Diagram](images/yii2-workflow-uml.jpg)