Add subdirectory for practicing design patterns in C++

+ Add example for singleton

cpp/patterns/singleton/main.cpp

@@ -0,0 +1,15 @@
+
+#include "singleton.h"
+
+int main(const int argc, const char *argv[])
+{
+  // Creates a singleton, initializes message in ctor
+  Singleton &s = Singleton::getInstance();
+  s.showMessage();
+  // Update already existing Singleton message, show it
+  s.updateMessage("First update\n");
+  Singleton::getInstance().showMessage();
+  // Update already existing Singleton message, show it
+  Singleton::getInstance().updateMessage("Second update\n");
+  s.showMessage();
+}