Add all 23 GoF design pattern implementations

03-behavioral/state/YellowState.java

@@ -0,0 +1,11 @@
+package state;
+
+public class YellowState implements TrafficLightState {
+    @Override public void onEnter(TrafficLight light) {
+        System.out.println("  [YELLOW] Slow down. Prepare to stop.");
+    }
+    @Override public void next(TrafficLight light) {
+        light.setState(new RedState());
+    }
+    @Override public String getColor() { return "YELLOW"; }
+}