Add README.

Author: ilopX

patterns/facade/conceptual/README.md

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+# Facade pattern
+Facade is a structural design pattern that provides a simplified interface to a library, a 
+framework, or any other complex set of classes.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/facade).
+
+### 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/facade/example)
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/183629745-a62d81be-f640-48e8-b70c-00d6cf59aa5f.png)
+
+### Client code:
+```dart
+void main() {
+  final converter = VideoConversionFacade();
+  final mp4Video = converter.convertVideo("youtubevideo.ogg", "mp4");
+  // ...;
+}
+```
+
+### Output:
+```
+VideoConversionFacade: conversion started.
+CodecFactory: extracting ogg audio...
+BitrateReader: reading file...
+BitrateReader: writing file...
+AudioMixer: fixing audio...
+VideoConversionFacade: conversion completed.
+```