Add README.

Author: ilopX

patterns/command/conceptul/README.md

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+# Command Pattern
+Command is a behavioral design pattern that turns a request into a stand-alone object that contains
+all information about the request.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/command).
+
+## Diagram:
+![image](https://user-images.githubusercontent.com/8049534/183062798-c2e9207d-850c-47b6-bbba-3d669299d69f.png)
+
+## Client dode:
+```dart
+void main() {
+  final mutStr = MutStr();
+
+  final input1 = AddTextCommand('One', mutStr);
+  final input2 = AddTextCommand('Three', mutStr);
+  final input3 = InsertTextCommand(' Two ', mutStr, pos: 2);
+
+  input1.execute();
+  print('text = $mutStr'); // mutStr = "One"
+
+  input2.execute();
+  print('text = $mutStr'); // mutStr = "OneThree"
+
+  input3.execute();
+  print('text = $mutStr'); // mutStr = "One Two Three"
+
+  input3.undo();
+  print('text = $mutStr'); // mutStr = "OneThree"
+
+  input2.undo();
+  print('text = $mutStr'); // mutStr = "One "
+
+  input1.undo();
+  print('text = $mutStr'); // mutStr = ""
+}
+```
+
+### Output:
+```
+text = One
+text = OneThree
+text = One Two Three
+text = OneThree
+text = One
+text = 
+```