doc: UML diagram

Author: dancergraham

chapter02_observer/readme.md

@@ -9,6 +9,40 @@ for instance, in Django.
 As I wrote out the code I found it very appealing that I did not need
 to change the subject at all to add new observers.
 
+### Class Diagram
+
+```mermaid
+
+classDiagram
+
+    Subject <-- Observer : observers
+    Subject1 <-- Observer1 : subject
+    Subject1 <-- Observer2 : subject
+    Subject <|-- Subject1
+    Observer <|-- Observer1
+    Observer <|-- Observer2
+    Subject : attach(o)
+    Subject : detach(o)
+    Subject: notify()
+    class Observer{
+      +update()
+    }
+    class Subject1{
+      state
+      get_state()
+      set_state()
+    }
+    class Observer1{
+        state
+        update()
+    }
+        class Observer2{
+        state
+        update()
+    }
+
+```
+
 ## Running the code
 
 ```bash