添加设计模式及例子，包括：单例、抽象工厂、适配器、桥接、观察者模式

DesignPattern/SingletonPattern/README.md

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+# 单例模式
+
+```cpp
+// 懒汉式单例模式
+class Singleton
+{
+private:
+	Singleton() { }
+	static Singleton * pInstance;
+public:
+	static Singleton * GetInstance()
+	{
+		if (pInstance == nullptr)
+			pInstance = new Singleton();
+		return pInstance;
+	}
+};
+
+// 线程安全的单例模式
+class Singleton
+{
+private:
+	Singleton() { }
+	~Singleton() { }
+	Singleton(const Singleton &);
+	Singleton & operator = (const Singleton &);
+public:
+	static Singleton & GetInstance()
+	{
+		static Singleton instance;
+		return instance;
+	}
+};
+```