Add README to example.

Author: ilopX

patterns/factory_method/concaptual_platform_dialog/README.md

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+# Abstract Factory pattern
+Factory Method is a creational design pattern that provides an interface for creating objects in a 
+superclass, but allows subclasses to alter the type of objects that will be created.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/factory-method).
+
+### About example.
+This the very conceptual example rewrite from original source code [java example](https://github.com/RefactoringGuru/design-patterns-java/tree/main/src/refactoring_guru/factory_method/example)
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/166105090-a2b490fe-3e3e-44f1-a781-9777023020fb.png)
+
+### Client code:
+```dart
+late Dialog dialog;
+
+void main() {
+  configure();
+  runBusinessLogic();
+}
+
+void configure() {
+  if (Platform.isWindows) {
+    dialog = WindowsDialog();
+  } else {
+    dialog = HtmlDialog();
+  }
+}
+
+void runBusinessLogic() {
+  dialog.renderWindow();
+}
+```
+
+### Output:
+```
+Windows Button
+Click! Button says - "Hello World!"
+```