Add README.

Author: ilopX

patterns/strategy/reservation_cargo_spaces/README.md

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+# Memento pattern
+Strategy is a behavioral design pattern that lets you define a family of algorithms, put each of 
+them into a separate class, and make their objects interchangeable.
+
+Tutorial: [here](https://refactoring.guru/design-patterns/strategy).
+
+## About example: Reservation cargo spaces. 
+This example is taken from the "Blue" book **"Domain-Driven Design" - Eric Evans. Chapter Once**.
+
+### Diagram:
+![image](https://user-images.githubusercontent.com/8049534/166560051-6e392b01-6777-4eb1-ae20-fcd643b248ef.png)
+
+### Client code:
+```dart
+void main() {
+  final overbookingPolicy = OverbookingPolicy();
+  final app = Application(overbookingPolicy);
+  final voyage = Voyage();
+
+  try {
+    app.makeBooking(Cargo(1000), voyage);
+    app.makeBooking(Cargo(500), voyage);
+    app.makeBooking(Cargo(800), voyage); // error
+  } catch (e) {
+    print(e);
+  }
+}
+
+class Application {
+  void makeBooking(Cargo cargo, Voyage voyage) {
+    if (overbookingPolicy.isAllowed(cargo, voyage)) {
+      voyage.addCargo(cargo, confirmation);
+    } 
+  }
+}
+```
+
+**Output:**
+```
+add Cargo(1000.0) to voyage.
+add Cargo(500.0) to voyage.
+The weight of the cargo exceeds the permissible norm.
+```