added command pattern example

src/learn/simple_command/Invoker.java

@@ -0,0 +1,13 @@
+package learn.simple_command;
+
+public class Invoker {
+    private Command slot;
+
+    public void setSlot(Command slot) {
+        this.slot = slot;
+    }
+
+    public void buttonWasPushed() {
+        slot.execute();
+    }
+}