added strategy example

src/learn/simple_strategy/Main.java

@@ -0,0 +1,11 @@
+package learn.simple_strategy;
+
+public class Main {
+    public static void main(String[] args) {
+        Context context = new Context(new ConcreteStrategyB());
+
+        context.doSomeJob();
+        context.setCurrentStrategy(new ConcreteStrategyA());
+        context.doSomeJob();
+    }
+}