add polymorphism example

src/encapsulationInheritancePolymorphism/polymorphism/ElectricEngine.java

@@ -0,0 +1,13 @@
+package encapsulationInheritancePolymorphism.polymorphism;
+
+public class ElectricEngine extends Engine {
+    private String batteryType;
+
+    public int getPower() {
+        return 20;
+    }
+
+    /*private class Engine {
+        private int power;
+    }*/
+}