#1317 Special Case Pattern (#1624)

* #1317 Add Special Case Pattern

To focus on pattern itself, I implement DB and
maintenance lock by the singleton instance.

* #1317 Add special cases unit tests

Assert the logger output
(ref: https://stackoverflow.com/a/52229629)

* #1317 Add README.md

Add Special Case Pattern README

* #1317 Format: add a new line to end of file

Co-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com>

special-case/src/main/java/com/iluwatar/specialcase/ApplicationServicesImpl.java

@@ -0,0 +1,18 @@
+package com.iluwatar.specialcase;
+
+public class ApplicationServicesImpl implements ApplicationServices {
+
+  private DomainServicesImpl domain = new DomainServicesImpl();
+
+  @Override
+  public ReceiptViewModel loggedInUserPurchase(String userName, String itemName) {
+    if (isDownForMaintenance()) {
+      return new DownForMaintenance();
+    }
+    return this.domain.purchase(userName, itemName);
+  }
+
+  private boolean isDownForMaintenance() {
+    return MaintenanceLock.getInstance().isLock();
+  }
+}