Add README.

Author: ilopX

patterns/observer/app_observer/README.md

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+# Observer pattern
+Observer is a behavioral design pattern that lets you define a subscription mechanism to notify 
+multiple objects about any events that happen to the object they’re observing.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/observer).
+
+## AppObserver example 
+This example was created to be used in a more complex example.
+A complex example will be implemented later.
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/152049751-b111e02a-1d33-4796-810c-b7ed069cecdc.png)
+
+### Sequence
+![image](https://user-images.githubusercontent.com/8049534/152049996-72131655-402d-4b92-b5d0-10e3f2dd0e79.png)
+
+### Client code:
+```dart
+void main() {
+  final observer = AppObserver();
+
+  observer.subscribe<FirstEvent>((e) {
+    print('First');
+  });
+
+
+  observer.subscribe((SecondEvent e) {
+    print('Second');
+  });
+
+  final saveThirdEvent = observer.subscribe((ThirdEvent e) {
+    print('Third');
+  });
+
+  observer.notify(FirstEvent());
+  observer.notify(SecondEvent());
+  observer.notify(ThirdEvent());
+
+  print('---unsubscribe "ThirdEvent"---');
+  observer.unsubscribe(saveThirdEvent);
+
+  observer.notify(FirstEvent());
+  observer.notify(SecondEvent());
+  observer.notify(ThirdEvent());
+}
+```
+
+**Output:**
+```
+First
+Second
+Third
+---unsubscribe "ThirdEvent"---
+First
+Second
+```