Add README.

Author: ilopX

patterns/abstract_factory/tool_panel_factory/README.md

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+# Abstract Factory Pattern
+Abstract Factory is a creational design pattern that lets you produce families of related objects 
+without specifying their concrete classes.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/abstract-factory).
+
+### Online demo:
+Click on the picture to see the [demo](https://RefactoringGuru.github.io/design-patterns-dart/#/abstract_factory/tool_panel_factory).
+
+[![image](https://user-images.githubusercontent.com/8049534/168668992-369a1bab-9f97-4333-a20e-ffd06bf91b54.png)](https://refactoringguru.github.io/design-patterns-dart/#/abstract_factory/tool_panel_factory)
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/168672053-73ae1c9c-8fad-45ae-9247-429f7b5da565.png)
+
+### Client code (using the "createShape" method):
+```dart
+final app = App(
+  tools: Tools(
+    factories: [
+      TxtFactory(),
+      LineFactory(),
+      CircleFactory(),
+      TriangleFactory(),
+      StarFactory(),
+    ],
+  ),
+);
+
+class App {
+  void addShape(double x, double y) {
+    final newShape = activeFactory.createShape(x, y, activeColor);
+    shapes.add(newShape);
+  }
+}
+
+mixin IconBoxMixin implements FactoryTool {
+  Image? _icon;
+
+  @override
+  Image get icon => _icon!;
+
+  Future<void> updateIcon(Color color) async {
+    final shape = createShape(0, 0, color);
+    final pngBytes = await _pngImageFromShape(shape);
+    _icon = Image.memory(pngBytes);
+  }
+}
+```
+