added headshots and enemy's shooting

Assets/Scripts/EnemyHeadHitbox.cs

@@ -0,0 +1,10 @@
+using UnityEngine;
+
+/// <summary>
+/// Attach this to an enemy's head GameObject.
+/// When the player shoots it, SimpleGun checks for this component and instakills.
+/// </summary>
+public class EnemyHeadHitbox : MonoBehaviour
+{
+    [HideInInspector] public EnemyHealth enemyHealth;
+}