Add Thorns to DamageCause - Related to BUKKIT-3505

By: riking <rikingcoding@gmail.com>

paper-api/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java

@@ -188,6 +188,12 @@ public class EntityDamageEvent extends EntityEvent implements Cancellable {
          * Damage: variable
          */
         FALLING_BLOCK,
+        /**
+         * Damage caused in retaliation to another attack by the Thorns enchantment.
+         * <p />
+         * Damage: 1-4 (Thorns)
+         */
+        THORNS,
         /**
          * Custom damage.
          * <p />